1902 Encyclopedia > Pacific Ocean

Pacific Ocean




Pacific Ocean. The ancient world was ignorant of the existence of the vast expanse of water now known as the Pacific Ocean. In Ptolemy's map of the world, constructed in the 2d century of our era (see MAP, vol. xv. PI. VII.), this fact is clearly brought out, for the only space which might possibly "represent the Pacific is the Magnus Sinus, a sea so limited in extent, and represented in such a position, that it probably stands for the Gulf of Siam in the Indian Ocean.

Vague reports of a great ocean lying beyond China Progres were current in Europe as early as the period of Arabian of supremacy in learning. Indeed an Arab merchant named Sulaiman, who visited China in the 9th century, declared that he had sailed upon it. But for several hundred years the reports continued so uncertain, and were so loaded with the wild extravagance of travellers' tales of the period, that it is difficult to get at the facts from which they probably took their origin. During the 13th and 14th centuries Marco Polo and his successors travelled far to the East and came to an. ocean of the extent of which they were ignorant, but they partially explored its western coasts. The East was the region towards which all the commerce and enterprise of the Middle Ages tended, and it was the hope of finding a safer and shorter sea route to India that led the Spanish court in 1492 to furnish Columbus with a fleet for the exploration of the Western Ocean. Although convinced of the spherical form of the earth, he greatly under-rated its size, and, accepting the popular estimate of the great breadth of the Asiatic continent, he set out on his voyage confident of soon reaching "the Indies." The glowing descriptions of his discoveries in that strange new world of the West that rose up before him to bar his advance immediately attracted the attention of adventur-ous Spanish mariners. Headed by Columbus himself, they cruised intrepidly amongst the Caribbean Islands, still lured by the hope of discovering some western passage to the coveted East. Columbus found that what he at first con-sidered a labyrinthine archipelago was a continent of vast extent, but not Asia, and he died without knowing what lay beyond. Spain and Portugal were the rival maritime powers at that time, and both took up the search for new countries with great ardour. Pope Alexander VI., in 1493, fearing that the two nations would quarrel over their colonies, assigned all the new lands that might be discovered west of the Azores to Spain, and all east of those islands to Portugal. The Portuguese accepting the gift followed Vasco da Gama in opening up the road to India by the Cape of Good Hope, and pushed forward their trading and piratical excursions into the west Pacific far beyond the Spice Islands. The Spaniards confined themselves to the New World, visiting, naming, and plundering the West India Islands and the headlands of Central America. On the 29th of September 1513 Vasco Nunez de Balbao, the leader of a Spanish party exploring the Isthmus of Panama, saw, from the summit of a mountain, a vast ocean stretching to the west—the very ocean of whose existence Columbus was certain, and which he had so long tried vainly to discover. Because he first saw it on Michaelmas day, Balbao named it the Go/fo de San Miguel. Magellan, following the east coast of America farther to the south than any previous explorer, sailed on, in spite of terrific storms, until he found the strait which now bears his name, and, steering carefully through it, on the 27th of November 1520 he swept into the calm waters of that new sea on which he was the first to sail, and which he named the Mar Pacijico.

The victories of Cortez in Mexico about the same date opened the way fer the exploration of the west coast of America, where Pizarro's conquest of Peru in 1526 gave the Spaniards a firm footing. From this time an inter-mittent trade sprang up between Europe and the Pacific through Magellan Strait, and latterly round Cape Horn. Before long English fleets, attracted more by the prospects of plundering Spanish galleons than of discovering new territories, found their way into the Pacific. Sir Francis Drake, like Balbao, saw the ocean from the Isthmus of Panama. He entered the Pacific in September 1577, being the first Englishman to sail upon it; some months later he sailed across it to the Moluccas. Alvaro de Mardana, who preceded him, had discovered the Solomon Islands in 1567.

Tasman, Roggewein, Dampier, and other explorers of the 17th century discovered Australia, New Zealand, Tasmania, and many smaller groups of islands. During the 18th century the voyages of Anson, Bass, Behring, the two Bougainvilles, Broughton, Byron, Cook, La Perouse, and many more practically completed the geographical explora-tion of the Pacific Ocean. In the beginning of that century the Pacific had a curious fascination for commercial speculators, and the ill-fated Scottish colony founded at Darien in 1698 seemed only to prepare the way for the English South Sea bubble that burst in 1720. All the navigators who explored these seas believed in the existence of a north-west passage between the Atlantic and Pacific, and made attempts to find it; but its discovery baffled all enterprise until 1850, when Maclure proved that there was such a channel, but that the ice prevented its being of any commercial utility. In the present century D'Entrecasteaux, Krusenstern, Beechy, Fitzroy, and Bennet have taken the lead amongst geographical explorers in the Pacific, although the ranks contain many names scarcely less worthy of remembrance. Within recent years several purely scientific exploring expeditions and British survey-' irig vessels have examined the Pacific, investigating its depth, the nature and form of the bottom, the tempera-ture of the water at various depths and its density, as well as the marine fauna and flora. Of those expeditions the voyages of the " Challenger," " Gazelle," and " Tuscarora " are the most important.

Extent.—The Pacific Ocean is bounded on the N. by Extent. Behring Strait and the coasts of Russia and Alaska, on the E. by the west coasts of North and South America; on the S. the imaginary line of the Antarctic Circle divides it from the Antarctic Ocean, while its western boundary is the east coast of Australia, the Malay Archipelago separating it from the Indian Ocean, and the eastern coasts of the Chinese empire. Some modern geographers place the southern limit of the Atlantic, Pacific, and Indian Oceans at the 40th parallel, and name the body of water which surrounds the earth between that latitude and the Antarctic Circle the Southern Ocean.

Although differing from the Atlantic in its general form, being more nearly land-locked to the north, the Pacific resembles it in being open to the south, forming, in fact, a great projection northwards of that vast southern ocean of which the Atlantic is another arm.

The Pacific is the largest expanse of water in the world, covering more than a quarter of its superficies, and com-prising fully one-half of its water surface. It extends through 132 degrees of latitude, in other words, it measures 9000 miles from north to south. From east to west its breadth varies from about 40 miles at Behring Strait, where Asia and America come within sight of each other, to 8500 miles between California and China on the Tropic of Cancer, and to more than 10,000 miles on the Equator between Quito and the Moluccas, where the ocean is widest. The area has been variously estimated at from 50,000,000 to 100,000,000 square miles; but, defining its boundaries as above, Keith Johnston, from careful measure-ments, estimated it, with probably a near approach to the truth, at 67,810,000 square miles.

Coasts, Seas, &c.—The coast-line of the Pacific and Indian Oceans, taken together, only amounts to 47,000 miles; that of the Atlantic alone measures 55,000, the smaller ocean more than making up for its less extent by its numerous inland seas and inlets of smaller size. Ameri- Speaking broadly, the eastern boundary of the Pacific is can rugged, barren, mountainous, and singularly free from coast. indentations, while its western shores are low, fertile, and deeply indented with gulfs and partially enclosed seas. Behring Strait unites the Arctic Ocean with the Sea of Kamchatka, or Behring Sea, which is bounded on the east by the irregular, low, swampy shores of Alaska, and on the south by the Alaskan peninsula and the Aleutian Islands. Along British North America the coast is rugged, rocky, considerably indented, and, between the parallels of 50° and 60° N. lat., fringed with islands. The largest of these are Vancouver Island in the Gulf of Georgia, Queen Charlotte Island, Prince of Wales Island, and the islands of King George III.'s Archipelago. The Gulf of California runs northwards in the Mexican coast, reach-ing from 23° to 32° N. lat. It is the one important inlet on the whole west coast of America,—the only others which are worth naming being the Gulf of Panama and the Gulf of Guayaquil. The Mexican shore is low, and contrasts with the coasts to the north and to the south, which are generally steep and rocky, though there are occasional sandy beaches in Peru and Chili. The breadth of the plain between the Rocky Mountains and the sea gradually diminishes towards the south, and the mountain chain of the Andes runs close along the west coast of South America to the very extremity of the con-tinent.

A series of volcanoes, active and extinct, runs round the Pacific, commencing at Cape Horn, passing along the Andes and Rocky Mountains, crossing from the American continent by the Aleutian Islands to Kamchatka, and thence southwards by Japan and the East Indian Archipelago to New Zealand. Earthquakes are frequent all along this line.

There are few islands near the American coast north of Patagonia, and these are small and unimportant; but south of the 40th parallel there is a complete change. The end of the continent seems as if it had been shattered ; there are abrupt bays and jagged chasms; archipelagos of small islands rise up in splintered fragments along the shore. The Strait of Magellan forms a tortuous channel between the mainland and the rocky storm-beaten islands of Tierra del Fuego.

Asiatic coast. The coast-line on the Asiatic side is longer and greatly diversified. In the north the Sea of Okhotsk is cut off from Behring Sea by the peninsula of Kamchatka, from the extremity of which a chain of islands extends to the borders of the Antarctic Ocean. These islands are of all sizes, ranging from small islets to the island continent of Australia. The island chain hangs in loops along the Asiatic coast, each loop including an almost land-locked sea. These partially enclosed seas are more or less com-pletely cut off from the general oceanic circulation, and they consequently differ considerably from the open ocean as regards the temperature of the water, specific gravity, fauna and flora, and nature of the deposits. The Kurile Islands run from Kamchatka to Japan, cutting off the Sea of Okhotsk. The great Japanese Islands, with Saghalien to the north and the Chinese coast on the west, enclose the Sea of Japan, leaving it in communica-tion with the Sea of Okhotsk by the Channel of Tartary to the north, with the ocean on the west by the Straits of La Perouse and Sangar, and on the south by the Straits of Corea. The Yellow Sea runs into the Chinese coast, and is divided from the Sea of Japan by the peninsula of Corea. The China Sea, with the two great gulfs of Tonquin and Siam, is marked off from the Indian Ocean by the peninsula of Malacca—remarkable because it runs in the same direction as the other two peninsulas of the Pacific, Kamchatka and Corea—and the islands of Sumatra and Java, while Borneo and the Philippine Islands separate it from the Pacific. Between the south coast of China and the north of Australia the East Indian Archi-pelago cuts up the .ocean into a network of small seas and narrow channels. The seas are named the Celebes, the Banda, the Sulu, the Java, the Flores, and the Arafura. The more important of the sea passages between the islands are the Straits and Channel of Formosa, which lead north-ward from the Pacific to the China Sea; the Strait of Macassar between Borneo and Celebes; Molucca Passage between Celebes, the Moluccas, and Jilolo; and Torres Strait between New Guinea and Australia. The east coast of Australia is, as a rule, steep and rocky; there are few inlets, and none of them compare in size with the Gulf of Carpentaria on the north coast. Moreton Bay and Port Jackson are two of the best harbours, and as a haven the latter has few equals in the world. The Great Barrier Reef lies off this coast for a length of more than a thousand miles, the distance between it and the shore varying from 60 to 100 miles. Bass Strait separates Australia from Tasmania on the south; and the two main islands of New Zealand, separated by Cook Strait, lie to the south-east of the continent. The Gulf of Hauraki, the Bay of Plenty, and Pegasus Bay are the chief inlets in these islands.
River-System.—The drainage area of the Pacific Ocean is estimated at 8,660,000 square miles, while that of the Atlantic amounts to more than 19,000,000; the chief reason for this disparity is that only half a million square miles of the American continent are drained into the Pacific, the remaining six and a half millions being con-nected with the Atlantic river-system, and it is estimated that only one-seventh of the area of the Asiatic continent drains into the Pacific Ocean. The huge wall of the Andes Ameri-practically reduces the Pacific rivers of South America to the c™ rank of mountain streams; the Biobio and the Maypu in "^"m Chili are the only ones exceeding 100 miles in length,— the former having a course of 180, the latter of 160 miles. The Rocky Mountain chain, which forms the watershed of North America, runs parallel to the Pacific coast at a distance of about 1000 miles, and the Cascade and minor ranges which skirt the shore are broken through in several places to give passage to rivers that are, in some cases, of considerable size. The Colorado rises in the State of that name, at the base of the Rocky Mountains, flows south-west through Utah and Arizona, and falls into the head of the Gulf of California. Its course measures about 1100 miles, and it drains a rugged and barren area of 170,000 square miles. California has only one river, the Sacramento, 420 miles long. The Oregon (or Columbia) is formed by the union of two streams rising in the Rocky Mountains, one in British Columbia, the other in Idaho. It is a swift-flowing river, full of rapids and cataracts, and, though it is only about 1000 miles long, the area which it drains is much greater than that drained by the Colorado. The ebb and flow of the tide are perceptible for a hundred miles from the mouth of the Oregon, and the river is navigable for that distance. The Frazer, which has a length of 600 miles, flows southward through British Columbia from the Bocky Mountains, and enters the sea in the Gulf of Georgia opposite Vancouver Island, carrying off the rainfall of 98,000 square miles. The northern limit of the American mountain chains is marked by the rise of the great river Yukon, which traverses Alaska; and, after a run of more than 2000 miles, it enters Behring Sea opposite the island of St Lawrence. Its tributaries have not been fully explored, so the area which they intersect is unknown, but probably it is very large. Asiatic The Asiatic division of the Pacific river-system is very river- much more extensive than the American, and includes system. many gtreams of great size and of considerable commercial importance. In the north the Amur is more than 2000 miles long, and it receives many tributaries, which rise on the north in the Stanovoi mountains, and on the west and south on the borders of the great table-land of the Gobi, the central Asiatic desert; altogether its basin measures nearly 900,000 square miles. The Hoang-ho (Hwang-ho or Whang-ho) and the Yangtze-keang both rise near the Kuen-lun mountains of Tibet amongst the extensive terraces which form the eastern slope of the great table-land of Central Asia. The Hoang-ho has a length of 2600 miles, and in its course it sweeps in a northerly curve close to the In-Shan mountains; then, after being crossed repeatedly by the Great Wall of China, it turns sharply to the south, and finally runs due east into the Yellow Sea. The Yangtze-keang follows a southward direction from its source, but ultimately turns to the north-east and enters the Yellow Sea not far from the mouth of the Hoang-ho. It is one of the longest rivers in the world, for, including its windings, it measures 3200 miles from its source to the sea. These two rivers drain more than a million and a quarter square miles; and it is principally owing to the large amount of suspended matter which they carry down that the sea into which they fall is called the Yellow Sea. The other rivers of importance are the Choo-keang, the Mekong, and the Menam. The last two run into the Gulf of Siam, after watering the peninsula of Siam and Cochin China. Few rivers enter the Pacific on the east coast of Australia, and in conse-quence of the proximity of the mountains to the shore they are short and unimportant. Atmo- Atmospheric Pressure and Prevailing Winds.—When the spheric mean atmospheric pressure for the year over the entire pressure. surface of the world is considered, it is found that there are two broad belts of high pressure which encircle the globe, one on each side of the equator. There is a wide area of slowly diminishing pressure between them, includ-ing a narrow central band along which the barometric readings attain a minimum. Two other regions of low pressure surround the poles, and extend to a considerable distance. That around the North Pole is connected with an area of still lower pressure over the North Pacific, and there is another permanent depression, which is even deeper, in the vicinity of Iceland. Atmospheric pressure is the fundamental meteorological phenomenon, and the mean pressure for the year affords a clue to the cause of all such regular and continuous phenomena as trade winds and ocean currents, and to the distribution of temperature. Similarly a study of the isobars at different seasons throws light upon all periodical occurrences in the way of winds and currents.

Prevailing winds. A low barometer is always accompanied by a high percentage of atmospheric aqueous vapour; consequently the equatorial belt of continuous low pressure is a region of almost continuous rain, excessive cloud, and constant calm or light variable winds. The effect of a difference in atmospheric pressure being established between two places is to produce a flow of air from the region of high towards that of low pressure, and the winds in their turn largely determine the surface movements or drift currents of the ocean. The region of calms between the north and south trades in the Pacific is both narrower, more irregular, and less clearly marked than the corresponding belt in the Atlantic. In the East Pacific it lies, at all seasons, con-siderably north of the equator; but during the southern summer it is found south of the line in the western parts of the ocean, and disappears entirely in the northern summer, as the calms of the Indian Ocean do also. The reason of the southern position of the west end of the calm belt seems to be the simultaneous occurrence of low atmo-spheric pressure in the interior of Australia and an ex-ceptionally high barometer in Asia. In the southern winter the depression over Asia and the increase of pressure over Australia form an unbroken barometric gradient, and the result is that the calms are replaced by a southerly breeze of great regularity. The region of calms included between the zones of the two trade winds, and towards which they blow, is not the only one with which they are associated ; for the opposite meteorological conditions that characterize the northern border of the north-east trades and the southern margin of the south-east winds produce two fringing bands of calms. These regions are characterized by a high barometer, a sunny sky, and' occasionally sudden squalls,—contrasting with the depressed barometer and dull, wet weather of the equatorial region. In January the low atmospheric pres-sure over the North Pacific produces winds which affect the climatological conditions of the shores in very different ways. At Vancouver Island the prevailing wind is south-west, and consequently the winter on the shores of British Columbia is mild and moist. The opposite coast of Asia is visited during the same season by northerly winds,— north-east in Alaska, north-north-east in Kamchatka, and north-west in Japan; and, as a result, the weather in these regions in winter is dry and bitterly cold. The West Pacific and the Indian Ocean are the regions of monsoons,— winds that blow as steadily as the trades, but which change their direction with the season. During the periods of transition the steady breeze gives place to variable winds, occasional calms, and sometimes terrific hurricanes. The general direction of the monsoons in the Pacific between April and October is southerly and south-easterly, and from November to April they blow from the north-east, and on nearing the continent of Asia from the north-west. Monsoonal winds are found connected with all continents; they are produced by the great differences in the tempera-ture and pressure which prevail over the land at different seasons as compared with the adjacent ocean. The mon-soons give rise to oceanic currents which flow in the same direction as the wind, and like it run opposite ways during alternate half years. Although the velocity of the wind over the open sea is always greater than that near shore or on land, it was shown by the observations of the " Chal-lenger," in the Pacific and other oceans, that there is no distinct diurnal variation in the wind's force at sea, though very decided periods of maxima and minima were noticed in the vicinity of land (see METEOROLOGY, vol. xvi. p. 125).

Currents.—The system of surface circulation in the Currents. Pacific is much more complicated and less clearly defined than that in the Atlantic, as might be expected from the less constant character of the winds. The latter ocean has two wide channels of communication with the Arctic Sea, while, so far as currents are concerned, the Pacific is land-locked to the north—Behring Strait being narrow and shallow; consequently water enters the Pacific almost entirely from the south, where there is uninterrupted communication with the Antarctic Ocean. There is no direct information as to the movements of ocean water at depths greater than 200 or 300 fathoms; it is known, however, from indirect evidence, that movements do occur. Although the subject of under-currents at depths less than those just mentioned has been extensively studied, it is only with respect to surface currents that anything very definite is as yet known

The vast extent of the Pacific Ocean gives full scope for the current-producing action of tides and winds, while the smooth continental boundary on its eastern side, the numerous groups of islands which break its surface, and its indented western coast, combine to modify the direction of the main streams and to produce innumerable minor currents, some permanent, and others varying from time to time in velocity and direction. The chief cause of these currents is believed to be traceable to the direct or indirect action of wind; but here it is proposed to refer merely to their general geography and physical effects, without dis-cussing the theory of their formation.

A general surface drift of the cold waters of the Antarctic Ocean, having a temperature lower than 40° Fahr. at all seasons, bears north-east towards Cape Horn, where it divides into two branches; one, the Cape Horn current, passes on into the Atlantic, and the other sweeps northward along the west coast of South America until it strikes the Peruvian shore, which deflects it westward. The cooling effect of this current on the water all along the coast is illustrated very clearly by the abrupt north-ward turn of the isothermals (see METEOROLOGY, figs. 8 and 9), which is more conspicuous in the chart for the southern winter than in that for the summer. In summer, however, there is a more striking evidence of this current's cooling power to be seen in the arrangement of the isothermals. The northern line of 70° Fahr. reaches as far south as 18° N. lat., and that of 80° makes a short loop from 18° N. to the equator; but the southern isothermal of 80° does not touch the American coast at all, and that of 70° lies farther from the equator than 30° S. lat., so that the increase of temperature from the south is very' gradual, so much so that at the Galapagos Islands, under the equator, the temperature of the surface water is only 70°, while a few hundred miles to the west it is over 80°. Penguins—essentially Antarctic birds—are found living on the shores of these islands. In consequence of this current, the highest surface temperature at all seasons of the year is found distinctly to the north of the equator in the eastern Pacific.

The Peruvian current forms the southern fork of the great equatorial current, which runs due west. This current is very broad, and divided by a narrow counter-current flowing in an opposite direction through its centre. The two branches of the equatorial current occupy very approximately the two areas of falling barometer between the north and south belts of high pressure and the central trough of minimum barometric readings. This difference of atmospheric pressure on each side produces the north-east and south-east trade winds, and to these the current probably owes its regularity and constant direction. The counter-current lies in the narrow belt of low barometric pressure to which the trades blow, and probably originates from the banking up of the waters to the westward. Its rate and position consequently vary greatly at different times of the year. The "Challenger," on her cruise between the Sandwich and Society Islands, found these currents to run with considerable force. In the " Narra-tive" of the cruise (chap, xviii.) the fact is alluded to thus:—

" From Hawaii Island to the 10th parallel the direction of the current was westerly, and its average velocity IS miles per day, ranging from 10 to 23 miles. From the 10th to the 6th parallel the direction was easterly, and its average velocity 31 miles per day, ranging from 7 to 54 miles per day. From the 6th parallel of north iatitude to the 10th parallel of south latitude the direction was again westerly, and the average velocity 35 miles per day, ranging from 17 to 70 miles per day. From thence to Tahiti the general tendency of the current was westerly, but its velocity was variable. The axis of greatest velocity of the counter-equatorial current was between the 7th and 8th parallels of north latitude. The axis of greatest velocity of the equatorial current was on tho parallel of 2° north, where its speed amounted to 3 miles per hour,"

The equatorial current strikes on the East Indian Archipelago, where it is split up by the narrow channels and shallow waters, and diverted into numberless minor currents. The two main divisions, which have acquired a high temperature from prolonged exposure to the tropical sun, ultimately leave the archipelago; the southern arm curves southwards, carrying its warm water to the east coast of Australia and to New Zealand, whence it is diverted towards the east, and becomes merged again in the general north-easterly antarctic drift. The north equatorial current, which varies in volume and velocity with the monsoons, strikes the coast of Asia between the Philippines and Japan, and is deflected in a north-easterly direction as the Kuro-Siwo or Japan current—wholly a warm oceanic river during the S.E. monsoon similar to the Gulf Stream of the Atlantic. The Japan current sends many branches into the inland seas and channels of the north-eastern coast of Asia, but the main body of water flows northward until it bifurcates in 40° N. lat., send-ing one fork among the Kurile Islands and along the Kamchatka peninsula into Behring Sea, whence it escapes by Behring Strait into the Arctic Ocean. A small counter-current of arctic water flows southward through Behring Sea, but it is not of sufficient volume to make its influence felt very decidedly on the general temperature of the surface water in the vicinity. The second and larger branch of the Japan current crosses the North Pacific, and, curving southward by Alaska and British Columbia, part of it returns as the north equatorial current, while the rest forms the variable Mexican current that runs along the coasts of California and Mexico.

The general direction of surface circulation in the Pacific may be remembered by supposing the ocean divided into a northern and southern half by the equatorial counter-current. In the northern half the water circulates in the direction of the hands of a watch, i.e., it passes up the west coast and down the east, while in the southern half the rotation is in the opposite direction—down the west coast and up the east; but the latter half does not exhibit the complete cycle so distinctly as the former. The centre of each area of circulation is occupied by a small Sargasso Sea, the northern being the more clearly defined, but neither approaches the well-known Sargasso Sea of the North Atlantic either in definiteness, extent, or amount of weed.

TEMPERATURE

Temperature of Surface Water.—The distribution of Surface temperature in the surface water of the Pacific varies considerably during the year. The equatorial region is ofture' course comparatively little affected by the change of season, but there is a general rise of temperature in the northern parts of the ocean, and a fall in the southern, during the northern summer, and a similar rise in the south and fall in the north during winter. The charts exhibit a general northward move in the isothermals during the former season, and a southward tendency in the latter. The change in the position of the lines is greatest in the temperate zones. The charts of ocean surface tempera-ture (see METEOROLOGY, figs. 8, 9) for February and August show the direction of the isothermals at two opposite seasons; and reference to them will make it plain that in temperate regions the lines of equal temperature follow the parallels of latitude much more closely in the Pacific than in the Atlantic, while their displacement with the change of season takes place in a direction nearly north and south. There are notable instances of divergence from these rules, such as the peculiarity of the isothermal of 80* already alluded to. Another circumstance is the fact that the temperature of the surface water on the western side of a great continent is much lower than that on the eastern side in the same latitude; it seems as if the west side of a continent attracted the isothermals, making them con-verge towards the equator. It has already been pointed out that these effects are due to the winds and the cold currents which strike the western continental shores and run along the coasts. The surface temperature of the Pacific, between the latitudes of 45° N. and 45° S., no-where at any season falls below 50°. In August the southern isotherm of 50° remains close to the 50th parallel, not diverging more than a degree or two on either side. Between the 45th parallels and the northern and southern limits of the ocean the temperature is almost always below 50°. The southern isotherm of 40° is remarkable for its constant position all the year round, between latitudes 55° and 58°,—a result brought about by the gigantic antarctic icebergs which prevent the surface temperature of the water from rising during the southern summer.

The northern and southern " isocrymes of 68°, that is the lines which pass over water which has a mean temperature of 68° during the coldest months of the year, lie, according to Dana (Corals and Coral Is/a.nds, 1872), between the latitudes of 20° and 30° on each side of the equator, except in the neighbourhood of the South-American coast, where the isocryme runs north in a loop beyond the equator,—a consequence of the cooling effect of the Peruvian current. These isocrymes mark out an area of great importance; for the reef-building corals are con-lined within it.

The highest temperature which sea water has been observed to attain is 90° F., and water of this temperature is only met with in the Red Sea. The maximum in the Pacific in the month of August is reached in the boundary between it and the Indian Ocean (in the Malay Archi-pelago) and in a narrow strip along the Mexican coast; in both these regions the thermometer immersed in the surface water registers 85° as a mean. There is a con-siderable area which in August stretches between New Guinea and Japan, from 10° S. to nearly 30° N., where the surface temperature reaches 84°, but these are excep-tional temperatures.

ture.

When the " Challenger" was cruising in the South Pacific—in 1874 and 1875—the water was found to bo uniformly warmer than the air, the difference in tem-perature between the two averaging 1°'5 to 2° Fahr. In the North Pacific, between the latitudes of 30° and 40°, on the other hand, the atmospheric temperature is about half a degree higher than that of the surface water. Such differences may be explained by considering the effect of warm and cold currents, which alter the temperature of the water much more rapidly than that of the air, and of warm and cold winds, which affect the atmosphere more quickly than the ocean. Deep-sea Deep-Sea Temperature.—The serial temperature sound-tempera- ings of the " Challenger " in the Pacific give a very good idea of the distribution of temperature in the deeper waters. There seems to be a slow massive movement of water from the Antarctic Ocean into the Pacific, which is not confined to the surface currents, but affects the whole mass of water down to the bottom. The rate of this motion is quite unknown. In the open sea, far from coasts and barriers, the temperature of the water con-tinually decreases as the depth increases. This is only true for the open ocean, fully exposed to the effects of the mass movement of the water; there is a very different distri-bution of temperature in enclosed seas such as those of the Western Pacific, or even in the ocean when a barrier pre-sents itself to the moving water. The difference, which is Plate II. brought out by the diagram (Plate II. fig. 1), is due to the aS- fact that when a barrier exists it retards the motion of the lower portion of the water, which has the lowest tempera-ture, while the higher passes on over it, and fills up the area beyond with water at the uniform temperature of the great ocean at the point to which the top of the ridge or obstruction reaches. In the Sulu Sea, for instance, the diagram shows that the temperature falls steadily and rapidly from 80° at the surface to 50° "5 at 400 fathoms, and then continues at 50°'5 right down to the bottom in 2500 fathoms, instead of sinking to somewhere about 35°, as it is observed to do in the open ocean at that depth. The inference is that the Sulu Sea is surrounded by a ridge rising to at least about 400 fathoms from the surface, which prevents the great ocean circulation from having its cooling effect, and soundings indicate that this is really the case. A study of the temperature phenomena, such as those just referred to, points out with considerable certainty the existence and height of barriers and ridges in many parts of the ocean, where their presence has not been detected by actual soundings.





During the cruise of the " Challenger" the bottom temperature over the North Pacific was found to be 35°T; south of the Sandwich Islands it fell to 35°; in the Low Archipelago it again rose to 35°T; on the 40th parallel it fell to 34°'7 in the deep water, but rose to 35°-4 and 35°"5 in the shallow water of the Patagonian elevation. The thermometer registered 34°"5 at the bottom between Australia and New Zealand; while in that part of the ocean to the north-east of Australia known as the Coral Sea, although the depth was the same (about 2500 fathoms), the bottom temperature was as high as 35°'9. The variations of temperature in the enclosed seas of the Eastern Archipelago were found to be considerable, and nearly all those seas show the phenomenon of constant temperature from an intermediate point to the bottom, consequent on the existence of barriers. The chief details of the thermal conditions of these seas are represented graphically in the diagram (Plate II. fig. 1). Between the Caroline Islands and Japan the bottom temperature was 35°'3. The bottom temperature in the Pacific is on the average about 1° F. lower than that in the Atlantic.

The temperature of the water at the depth of 300 fathoms is nearly the same (40° to 45°) over the whole of the North Pacific, but above 300 fathoms the water is warmer in the western than in the central portion, while below that depth it is colder in the former than in the latter. The same phenomenon is noticed between the latitudes of 34° S. and 40° S., but here 700 fathoms marks the plane of constant temperature. Between 33° N. and 40° S. the temperature of the water above 200 fathoms is higher in the North than in the South Pacific, whilst from 200 to 1500 fathoms it is lower in the North, and below the latter depth the condition reverts to what it was above 200 fathoms.

The diagram (Plate II. fig. 2) exhibits the bathymetrical Plate II. distribution of temperature in a section of the Pacific from a fig. 2. position in 38° 9' N. lat. and 156° 25' "W. long, to one in 40° 3' S. lat. and 132° 58' W. long, as determined by H.M.S. "Chal-lenger" in 1875, and may be compared with similar diagrams of the ATLANTIC (see vol. iii. p. 23). In order to separate the iso-thermals in the first 200 fathoms sufficiently the scale of depths required to be made large, while in order that the length of the diagram might be kept within reasonable bounds the scale of latitude was made very much smaller. The result of this is to exaggerate the inequalities of the sea bottom, making the slopes very much steeper than they are; this effect is best seen in the way in which islands are represented. The rapid falling off of temperature in the first few hundred fathoms, and then its very slow but steady decrease to the bottom are to be observed, and the fact that latitude has a great effect on the surface temperature, but none at considerable depths, for the isotherm of 40° is constantly between 300 and 400 fathoms, and also that depth alone deter-mines the bottom temperature in the open ocean, the coldest water occurring as a matter of fact under the equator in the deepest troughs open to the south.

Density of the Water.—The specific gravity of the water is an index of its salinity, since the researches of various chemists, foremost amongst whom are Forchham and Dittmar, have shown conclusively that the per-centage composition of the salts in sea water is the same in all parts of the ocean, so far at least as regards the principal constituents. Mr J. Y. Buchanan made continu-ous observations on the specific gravity of sea water during the whole voyage of the "Challenger," and has published a very valuable paper on the distribution of salt in the ocean in the "Challenger" Beports (Phys. Chem. Chall. Exp., Plate II. vol. i. part ii.). The chart in Plate II. showing the geogra-ohart. phical distribution of surface density is copied from that paper. The percentage of total salts in sea water, as deduced from the specific gravity, is, according to Buchanan and Dittmar—
Density 1-025 1026 1-027 1'028
Percentage 3'3765 3-5049 3'6343 37637

The density of the water in different parts of the ocean must obviously change to a certain extent with the season; and it is not only the surface density that is affected in this way; any cause which promotes evaporation tends to increase the salinity of surface water, while any con-ditions that effect condensation of aqueous vapour produce dilution. For instance, in the China Sea during the month of November, at the end of the south-west monsoon, which is a moist wind accompanied by much rain, the specific gravity observed was 1'02518, and two months later, after the dry north-east monsoon had been blowing for some time, evaporation had proceeded so far that the specific gravity had risen to 1-02534. The climate is the principal factor in determining surface salinity, and the causes which produce well-marked climatic conditions have an equally apparent effect on the density of the water. Thus there are two zones of comparatively high density encircl-ing the globe in the region of the north-east and south-east trade winds, which are dry and promote rapid evaporation; and similarly the region of calms and rain between the trades is distinguished by the low specific gravity of the water. North and south of these areas there are two zones where the salinity maintains a mean value, in consequence of there being a balance between evaporation and con-densation ; and round the poles there are areas of concentration brought about by the freezing of the sea water and the separation of salt, which of course increases the salinity of the water remaining unfrozen.

The distribution of density differs considerably in the two great oceans. In the Atlantic there are two areas of high specific gravity, one in the north, the other in the south; while in the Pacific there is only one, situated in the southern division of the ocean in the neighbourhood of the Society Islands. It is neither so large as those of the Atlantic, nor has it so high a specific gravity. The density of the concentration areas in the Atlantic, taking pure water at 4° C. as unity, is 1-02750; that in the saltest portion of the Pacific is only 1'02700. In the North Pacific the salinity is less than in the South, and its distribution is much more uniform. The density in this region never exceeds 1-02650, and the minimum, in the rainy region of the equatorial counter current, is as low as 1'02485. The South Pacific has water of a relatively high density, its maximum being 1'02750. The water of the seas of the Eastern Archipelago, in the western basin of the Pacific, although exposed to the full force of an equatorial sun, and possessed of a very high surface temperature, is yet surprisingly fresh. The specific gravity varies considerably with the season, but the aver-age for the year over the greater part of these seas is under 1 '02550; and there is a large area surrounding the islands of Java and Sumatra where the dilution is greater, the hydrometer only indicating 1'02500. The weak salinity of these waters is largely to be attributed to the extreme humidity of the atmosphere, the frequent and heavy rains, and the fact that so many lofty and exten-sive islands, where the annual rainfall rises above 200 inches, drain into the seas. Water of such a degree of dilution is not met with anywhere else, except near the mouths of rivers and in the vicinity of melting ice, and, as a temporary phenomenon, after prolonged rain in the tropics.

In regions where there is decided and continuous con-centration in progress, the specific gravity of the water is greatest at the surface and decreases as the depth increases, down to about 800 or 1000 fathoms, after which the density increases slowly with the depth until the bottom is reached. The density of the bottom water of the Pacific is almost the same everywhere; it only varies from T02570 to 1-02590; and the same value holds for the South Atlantic. The North Atlantic has denser water at the bottom, varying from F02616 to 1'02632. In those regions where the surface water is being constantly diluted, as is the case in the equatorial belt of calms, the density increases with the depth down to between 50 and 100 fathoms, where there is a maximum, from which the density diminishes, as in the other case, to about 1000 fathoms, and afterwards increases slowly down to the bottom. There is a striking resemblance between the direction of the isohalsines, or lines of equal salinity, and of the isothermals; but the parallelism breaks down, of course, in the case of a subsurface maximum.

Depth.—For a long time the opinion that the Pacific \was a comparatively shallow ocean was entertained by geographers, and it is only the recent soundings of the " Challenger," " Tuscarora," " Gazelle," and other survey-ing ships that have succeeded in dispelling the illusion. It is now known that the average depth of the Pacific is greater than that of the Atlantic, and that areas of deeper water occur in it than in any other part of the globe. A line running along the western shores of the two Americas and along the eastern shores of the Asiatic continent more or less closely follows a great circle of the globe. On the one side of this line there are the continental masses of the Americas and of Europe and Asia, with an average height of about 800 feet above the level of the sea; and on the other side the vast oceanic depression of the Pacific, with an average depth of about 2500 fathoms. The average level of the continental area may thus be regarded as about three miles above the Pacific depression.

The attempt to divide the ocean into sharply defined basins is more or less unsatisfactory; and for the considera-tion of the depth it is better to view the Pacific as marked off into two portions by an imaginary line passing through Honolulu and Tahiti, on the meridian of 150° W.

The eastern half is remarkable for the comparative absence of islands and the uniform nature of its depth. With the exception of the narrow strip of shallow water surrounding the Aleutian Islands and running along the American coast, the sounding line shows an average depth of from 2000 to 3000 fathoms undiversified by remarkable elevations or depressions, between the northern limit of the ocean and 30° S. lat. There is a great submarine plateau extending from the Patagonian coast (in 76° W. long.) in a westerly direction to 120° W. long., which rises to between 2000 and 1000 fathoms of the surface. This elevated area diminishes in breadth as it proceeds westward, but it is supposed by some authorities to be connected with the shallow water surrounding the Low Archipelago and the Marquesas Islands (groups which are bisected by the 140th meridian of west longitude) and the Society Islands. If this be the case there is an almost continuous area of elevation stretching between Patagonia and Japan. It has been remarked that many of the sub-merged plateaus of the Pacific have a south-east to north-west trend. The "Challenger" examined the depth of the eastern half of the Pacific in 1875, along a line which extended from 38° N. lat. on the 160th meridian south-east to the Sandwich Islands, and then as nearly as possible along the 150th meridian to the Society Islands in 23° S. lat. From this point the course was again south-east to the 40th parallel of south latitude, which was followed eastward to the Patagonian coast, a visit to Juan Fernan-dez forming a northward digression. The depth was ascertained at fifty points along this route, and it was found to vary on the whole from 2000 to 3000 fathoms. There were two soundings of over 3000 fathoms between latitudes 38° and 36° N., and one a little to the south of the Sandwich Islands. Between the meridian of 120° W. and the coast of America the soundings showed the depth to vary considerably as the ship was in deep water or over the submerged Patagonian plateau. The actual numbers observed proceeding eastward from 120° W. long, were in fathoms :—2250, 1600, 2025, 2270, 1500, 1825,1775,1375, 2160, 2225,1450,1325. The soundings made by the United States ship " Tuscarora " during 1874 were much more numerous, closer together, and extended along several lines, but the general result was similar to that of the " Challenger " observations. The results of all recent observations are shown on Plate III.

The western half of the Pacific Ocean is a complete con-trast to the eastern. Archipelagos and scattered islands are exceedingly numerous; the depth of the ocean is by no means uniform, for shallows and areas of unusual depth occur scattered over it at irregular intervals. Along the Asiatic coast and between the island groups there are a number of partially enclosed seas, and these are separated from the great ocean by submarine plateaus of sufficient extent and height to warrant the supposition that a moderate upheaval would extend the Asiatic continent as far south as Australia, transforming the seas into inland salt lakes. Considerations of the peculiar animal and vegetable life of New Zealand and Australia lend some degree of probability to the speculation that these islands were joined to the main continent of Asia at some very remote period; and it is even possible to trace the sub-merged coast-line of the great continent which then existed. This line separates the very deep water of the West Pacific from the shallower water of the inland seas and archipel-agos ; it runs from Kamchatka, over Japan, Formosa, the Philippines, New Guinea, to Australia and New Zealand. The most conspicuous peculiarity of the West Pacific is the very deep water lying in a crescent shape to the east of the Kurile Islands and Japan. It extends from 50° N. lat. to nearly 20° N. lat., although it is of no great breadth. The average depth of this area is nearly 4000 fathoms, and a narrow strip of still more abysmal depths runs along its western margin, like a ditch across the entrance to the Sea of Okhotsk ; here the United States ship " Tuscarora " found depths of over 4600 fathoms. The course of the " Challenger " led her to explore the seas of the Eastern Archipelago pretty thoroughly, and she carried a line of soundings from the archipelago to Japan, and thence east-ward across the Pacific, crossing the area of great depth about the centre, off Nippon, where two soundings of 3950 and 3625 fathoms respectively were obtained. Like the East Pacific, the western division of the ocean has an average depth of from 2000 to 3000 fathoms, although a great number of small depressions exist where the depth is greater, and detached areas of shallower water occur still more frequently. Many of the islands rise from depths of about 3000 fathoms, forming isolated mountains springing from the bed of the ocean, and several peaks which do not rise to the surface have been detected. More usually a number of islands are bound together by submarine elevations, frequently within a few hundred fathoms of the surface, over wide areas. Although the greater part of the sea surrounding New Zealand, the north of Australia, and the adjacent islands is under 1000 fathoms in depth, there are areas of great depression amongst the islands, and some very deep channels. In 1875 when sounding in the channel between the Carolines and Ladrones, the " Challenger " met with the deepest water of the cruise, 4475 fathoms, or about five miles and a quarter ; and this is the greatest depth from which a specimen of the bottom has hitherto been obtained. This abysmal depth only extends over a relatively small area, for the two nearest " Challenger " stations, one to the north and one to the south, had depths of 2300 and 1850 fathoms respectively.

The seas branching off from the Pacific are usually relatively shallow. Behring Sea on the north has ex-tremely shallow water in its north-eastern half, where there is a depth of under 100 fathoms ; in the south-western por-tion the depth increases rapidly to between 1000 and 2000 fathoms, except round the coasts and the Aleutian Archi-pelago. The Sea of Okhotsk is still shallower : much of it is within the 100 fathom line ; and in its deepest pjart it does not attain 1000 fathoms. The Yellow Sea is entirely within the hundred fathom line ; while the Sea of Japan, only separated from it by the Corean Peninsula, is not inferior in depth to the open ocean, its average depth being from 2000 to 3000 fathoms. The western portion of the Pacific, which lies between the Philippines and the Carolines and Ladrones, is also very deep, its mean depth approaching 3000 fathoms. This sea is of importance, since it is to the Pacific what the Gulf of Mexico is to the Atlantic—the source of its great northern thermal current. The fact that the temperature at 1500 fathoms over the whole of the North Pacific does not differ by more than 0°'5 F. from that at the bottom appears to indicate that this portion is cut off from the southern division by a ridge rising to within 1500 fathoms of the surface. The existence of such a barrier cannot be said to be proved, but the indications lead to the supposition that it may extend from Japan to the equator, through the Bonin, the Ladrone, and the Caroline Islands.

Taken altogether, so far as present knowledge goes, the bed of the Pacific is more uniform than that of the Atlantic, and its changes of level are less abrupt. Its depth is, on an average, greater, and appears to be more evenly distributed than in the Atlantic, but this appar-ent greater uniformity may be partly due to the fact that the latter ocean, both on account of its smaller size and its greater commercial importance, has been much more care-fully surveyed, and its bathymetrical conditions more exactly ascertained.

DEPOSITS.

Deposits. The explorations of the "Challenger," "Tuscarora," and other surveying ships have in recent years given a great amount of information respecting the nature of the deposits now forming over the floor of the ocean, and the specimens collected by these expeditions have been made the subject of a careful investigation by Messrs Murray and Renard. The great extent and depth of the Pacific Ocean make it the most suitable field for the study of the varieties of deep-sea deposits and the conditions under which they are found. The various kinds of deposits, all of which are found in the Pacifia Ocean, are classed as follows :—

Shore formations. Blue mud.
Green mud and sand. Red mud.
Pelagic deposits.
Coral mud and sand. Coralline mud and sand. Volcanic mud and sand. Red clay. Globigerina ooze. Pteropod ooze. Diatom ooze. Radiolarian ooze.

Terrigenous deposits. The terrigenous deposits are found in more or less close proximity to the land, and are chiefly made up of the triturated fragments carried down into the ocean by rivers, or worn away from the coasts by waves or currents. Those found in the deeper water surrounding the land differ from the sands, gravels, and shingles of the shore and shallow water chiefly in the smaller size of the grains and the greater abundance of clayey matter and remains of oceanic organisms. As, however, the water becomes still deeper and the distance from land greater, the deposits assume, more and more, a deep-sea character until they pass into a true pelagic deposit.

The principal mineralogical constituents of the terrigenous deposits near continental land are isolated fragments of rocks and minerals coming from the crystalline and schisto-crystalline series, and from the clastic and sedimentary formations; according to the character of the nearest coasts they belong to granite, diorite, diabase, porphyry, &c, crystalline schists, ancient limestones, and the sedimentary rocks of all geological ages, with the minerals which come from their disintegration, such as quartz, monoclinic and tri-clinic felspars, hornblende, augite, rhombic pyroxene, olivine, muscovite, biotite, titanic and magnetic iron, tourmaline, garnet, epidote, and other secondary minerals. The trituration and decomposition of these rocks and minerals give rise to materials more or less amorphous and without distinctive characters, but the origin of which is indicated by association with the rocks and minerals just mentioned.

Mixed with these are found in many places phosphatic nodules, large quantities of glauconite, and minerals arising from chemical action probably in presence of organic matter.

Blue mud. Blue mud is the most extensive deposit now forming around the great continents and continental islands, and in all enclosed or partially enclosed seas. It is characterized by a slaty colour, which passes in most cases into a thin layer of a reddish colour at the upper surface. These deposits are coloured blue by organic matter in a state of decomposition, and frequently give off an odour of sulphuretted hydrogen. When dried, a blue mud is greyish in colour, and rarely or never has the plasticity and compactness of a true clay. It is finely granular, and occasionally contains fragments of rocks 2 cm. in diameter; generally, however, the minerals which are derived from the continents, and are found mixed up with the muddy matter in these deposits, have a mean diameter of 0'5 mm. and less. Quartz particles, often rounded, play the principal part; next come mica, felspar, augite, hornblende, and all the mineral species which come from the disintegration of the neighbouring lands, or the lands traversed by rivers which enter the sea near the place where the specimens have been collected. These minerals make up the principal and characteristic portion of blue muds, sometimes forming 80 per cent, of the whole deposit. Glauconite, though generally present, is never abundant. The remains of calcareous organisms are at times quite absent, but occasionally they form over 50 per cent. The latter is the case when the specimen is taken at a considerable distance from the coast and at a moderate depth. These calcareous fragments consist of bottom-living and pelagic Foraminifera, Molluscs, Polyzoa, Serpulse, Echinoderms, Alcyonarian spicules, Corals, &c. The remains of Diatoms and Radiolarians are usually present. Generally speaking, as the shores are approached the pelagic organisms disappear ; and, on the con-trary, as we proceed seawards the size of the mineral grains diminishes, and the remains of shore and coast organisms give place to pelagic ones, till finally a blue mud passes into a true deep-sea deposit. In those regions of the ocean affected with floating ice, the colour of these deposits becomes grey rather than blue at great distances from land, and is further modified by the presence of a greater or less abundance of glaciated blocks and fragments of quartz. These deposits are found along the coasts of North and South America, and in all the enclosed and partially enclosed seas, such as the Japan Sea, China Sea, Arafura Sea, Sulu Sea, Banda Sea, Celebes Sea, Sea of Okhotsk, &c. Green At some points in the same regions are found green muds and muds sands, which, as regards their origin, composition, and distribution and near the shores of continental land, resemble the blue muds. They sands. are largely composed of argillaceous matter and mineral particles of the same size and kind as the blue muds. Their chief character-istic is the presence of a considerable quantity of glauconitic grains, either isolated or united into concretions by a brown argillaceous matter. The Foraminifera and fragments of Echinoderms and other organisms in these muds are frequently filled with glauconitic substance, and beautiful oasts of these organisms remain after treatment with weak acid. At times there are few calcareous organisms in these deposits, and at other times the remains of Diatoms and Radiolarians are abundant. When these muds are dried they become earthy and of a grey-green colour. They frequently give out a sulphuretted hydrogen odour. The green colour appears sometimes to be due to the presence of organic matter, probably of vegetable origin, and to the reduction of peroxide of iron to protoxide under its influence. The green sands differ from the muds only in the comparative absence of the argillaceous and other amorphous matter, and by the more important part played by the grains of glauconite, to which the green colour is chiefly due. Red mud is found where quantities Red of ochreous matter are brought down by rivers and deposited along mud. the coast, as in the Yellow Sea, but it is most characteristic in the Atlantic off the Brazil coast of America.

Volcanic muds and sands. In addition to the terrigenous deposits above referred to, volcanic muds and sands and coral are found around the muds and shores of oceanic islands either of volcanic or coral origin. The sands. volcanic muds and sands are black or grey, and when dried are rarely coherent. The mineral particles are generally fragmentary, and consist of lapilli of the basic and acid series of modern volcanic rocks, which are scoriaceous or compact, vitreous or crystalline, and usually present traces of alteration. The minerals are sometimes isolated, sometimes surrounded by their matrix, and consist principally of plagioclases, sanidine, amphibole, pyroxene, biotite, olivine, and magnetic iron ; the size of the particles diminishes with distance from the shore, but the mean diameter is generally 0'5 mm. Glauconite does not appear to be present in these deposits, and quartz is also very rare or absent. The fragments of shells and rocks are frequently covered with a coating of peroxide of manganese. Shells of calcareous organisms are often present in great abundance, and render the deposit of a lighter colour. The remains of Diatoms and Radiolarians are usually present.

Coral muds. Coral muds frequently contain as much as 95 per cent. of carbonate of lime, consisting of fragments of Corals, calcareous algae, Foraminifera, Serpulse, Molluscs, and remains of other lime- and secreting organisms. There is a large amount of amorphous sands, calcareous matter, which gives the deposit a sticky and chalky character. The particles may be of all sizes according to the distance from the reefs, the mean diameter being 1 to 2 mm., but occasionally there are large blocks of coral and large calcareous concretions ; the particles are white and red. Remains of siliceous organisms seldom make up over 2 or 3 per cent, of a typical coral mud. The residue consists usually of a small amount of argillaceous matter, with a few fragments of felspar and other volcanic minerals ; but off barrier and fringing reefs facing continents there may be a great variety of rocks and minerals. Beyond a depth of 1000 fathoms off coral islands the debris of the reefs begins to diminish, and the remains of pelagic organisms to increase ; the deposit becomes more argillaceous, of a reddish or rose colour, and gradually passes into a Globigerina, ooze or a red clay. Coral sands con-tain much less amorphous matter than coral muds, but in other respects they are similar, the sands being usually found nearer the reefs and in shallower water than the muds, except inside lagoons. In some regions the remains of calcareous alga? predominate, and in these cases the name coralline mud or sand is employed to point out the distinction.

The extent and peculiarities of the region in which these terrigenous deposits are laid down are interesting. It extends from high-water mark down, it may be, to a depth of over 4 miles, and in a horizontal direction from 60 to perhaps 300 miles sea-wards, and includes all inland seas, such as the North Sea, Norwegian Sea, Mediterranean Sea, Red Sea, China Sea, Japan Sea, Caribbean Sea, and many others. It is the region of change and of variety with respect to light, temperature, motion, and biological conditions. In the' surface waters the temperature ranges from 80° F. in the tropics to 28° F. in the polar regions. From the surface down to the nearly ice-cold water found at the lower limits of the region in the deep sea there is in the tropics an equally great range of temperature. Plants and animals are abundant near the shore, and animals extend in relatively great abundance down to the lower limits of the region, now marked out by these terrigenous deposits. The specific gravity of the water varies much, and this variation in its turn affects the fauna and flora. In the terrigenous region tides and currents produce their maximum effect, and these influences can in some instances be traced to a depth of 300 fathoms, or nearly 2000 feet. The upper or continental margin of the region is clearly defined by the high-water mark of the coast-line, which is constantly changing through breaker action, elevation, and subsidence. The lower or abysmal margin passes in most cases insensibly into the abysmal region, but may be regarded as ending where the mineral particles from the neighbouring continents begin to disappear from the deposits, which then pass into an organic ooze or a red clay.

The area covered by terrigenous deposits has been called the " transitional" or " critical area," and is estimated at about two-eighths of the earth's surface, while the continents cover three-eighths, and the deep-sea deposits of the abysmal regions, which will now be considered, cover the remaining three-eighths. Pelagic The true deep-sea deposits may be divided into two classes, viz., deposits, those in which the organic elements predominate, and those in which the mineral constituents play the chief part. Belonging to the former class there are Globigerina, Pteropod, Diatom, and Radiolarian Oozes, and to the latter Red Clay.

Globigerina ooze.

Globigerina ooze is the name given to all those truly pelagic deposits containing over 40 per cent, of carbonate of lime which con-sist principally of the dead shells of pelagic Foraminifera (Globige-rina, Orbulina, Palvinulina, Pullenia, Sphaeroidina) and coccoliths and rhabdoliths. In some localities this deposit contains 95 per cent, of carbonate of lime. The colour is milky white, yellow, brown, or rose, the varieties of colour depending principally on the relative abundance in the deposit of the oxides of iron and manganese. This ooze is fine grained ; in the tropics some of the Foraminifera shells are macroscopic. When dried it is pulverulent. Analyses show that the sediment contains, in addition to carbonate of lime, phosphate and sulphate of lime, carbonate of magnesia, oxides of iron and manganese, and argillaceous mat-ters. The residue is of a reddish-brown tinge. Lapilli, pumice, and glassy fragments, often altered into palagonite, seem always to be pre-sent, and are frequent-ly very abundant. The mineral particles are generally angular, and rarely exceed 0'08 mm. in diameter; mono-clinic and triclinic fel-spars, augite, olivine,

==

Fia. 1.—The finer particles of a Globigerina Ooze, hornblende, and mag- showing, Coccoliths, Coccospheres, and Rhab-netite are the most fre- aoliths.

Globigerina Ooze from 1900 fathoms.

==

quent. When quartz is present, it is in the form of minute, rounded, probably wind-borne grains, often partially covered with oxide of iron. More rarely there are white and black particles of mica, bronzite, actinolite, chromite, glauconite, and cosmic dust. Siliceous organisms are probably never absent, sometimes forming 20 per cent, of the deposit, while at other times they are only recognizable after careful microscopic examination. In some regions the frustules of Diatoms predominate, in other the skeletons of Radiolarians.

Pteropod ooze. Pteropod ooze differs in no way from a Globigerina ooze except in the presence of a greater number and variety of pelagic organisms, and especially in the presence of Pteropod and Heteropod shells, such as Diacria, Atlanta, Siyliola, Carinaría, &c. The shells of the more delicate species of pelagic Foraminifera and young shells are also more abundant in these deposits than in a Globigerina ooze. It must be remembered that the name " Pteropod ooze " is not intended to indicate that the deposit is chiefly composed of the shells of these Molluscs, but, as their presence in a deposit is char-acteristic and has an important bearing on geographical and bathy-metrical distribution, it is desirable to emphasize the presence of these shells in any great abundance. It may be pointed out that there is a very considerable difference between a Globigerina ooze or a Pteropod ooze situated near continental shores and deposits bearing the same names situated towards the centres of oceanic areas, with respect both to mineral particles and to re-mains of organisms.

Diatom ooze. Diatom ooze is of a pale straw colour, and is composed principally of the frustules of Diatoms. When dry it is a dirty white siliceous flour, soft to the touch, taking the impression of the fingers, and contains gritty particles which can be recognized by the touch. It contains on an average about 25 per cent, of carbo-nate of lime, which exists in the deposit in the form of small Glo-bigerina shells, frag-ments of Echinoderms and other organisms. The residue is pale white and slightly plastic ; minerals and fragments of rocks are in some cases abun-dant ; these are vol-canic, or, more fre-quently, fragments and minerals coming from continental rocks and transported by glaciers. The fine washings consist essen-tially of particles of Diatoms along with argillaceous and other amorphous matter. It is estimated that the frustules of Diatoms and skeletons of siliceous organisms make up more than 50 per cent, of this deposit.





It has been already mentioned that Radiolarians are seldom, if ever, completely absent from marine deposits. In some regions they make up a considerable portion of a Globigerina ooze, and are also found in Diatom ooze and in the terrigenous de-posits of the deeper water surrounding the land. In some regions of the Pa-cific, however, the skele-tons of these organisms make up the principal part of the deposit, to which the name Sadio-larian ooze has been given. The colour is reddish or deep brown, due to the presence of the oxides of iron and manganese. The mineral particles consist of fragments of pumice, lapilli, and volcanic mine-rals, rarely exceeding 0'07 mm. in diameter. There is not a trace of carbonate of lime in the form of shells in some samples of Radiolarian ooze, but other specimens contain 20 per cent, of carbonate of lime derived from the shells of pelagic Foraminifera. The clayey matter and mineral particles are the same as those found in the red clays, which will now be described.

Red clay. Of all the deep-sea deposits red clay is the one which is distributed over the largest areas in the modern oceans. It might be said that it exists everywhere in the abysmal regions of the ocean basins, for the residue in the organic deposits which have been described under the names Globigerina, Pteropod, Diatom, and Radiolarian oozes is nothing else than the red clay. However, this deposit only appears in its characteristic form in those areas where the terrigenous minerals and calcareous and siliceous organisms disappear to a greater or less extent from the bottom. It is in the central regions of the Pacific that the typical examples are met with. Like other marine deposits, this one passes laterally, according to position and depth, into the adjacent kind of deep-sea ooze, clay, or mud.

The argillaceous matters are of a more or less deep brown tint from the presence of the oxides of iron and manganese. In the typical examples no mineralogical species can be distinguished by the naked eye, for the grains are exceedingly fine and of nearly uniform dimensions, rarely exceeding 0'05 mm. in diameter. It is plastic and greasy to the touch ; when dried it forms lumps so coherent that considerable force must be employed to break them. It gives the brilliant streak of clay, and breaks down in water. The pyrognostic properties show that it is not a pure clay, for it fuses easily before the blowpipe into a magnetic bead.

Under the term red clay are comprised those deposits in which the characters of clay are not well pronounced, but which are mainly composed of minute particles of pumice and other volcanic material which, owing to their relatively recent deposition, have not under-gone great alteration. If the analyses of red clay are calculated, it will be seen, moreover, that the silicate of alumina present as clay (2Si02, A1203 + 2H20) comprises only a relatively small portion of the sediment; the calculation shows always an excess of free silica, which is attributed chiefly to the presence of siliceous organisms.

Microscopic examination shows that a red clay consists of argillaceous matter, minute mineral particles, and fragments of siliceous organisms. The mineral particles are for the greater part of volcanic origin, except in those cases where continental matters are transported by floating ice, or where the sand of deserts has been carried to great distances by winds. These volcanic minerals are the same constituent minerals of modern eruptive rocks enum-erated in the description of volcanic muds and sands ; in the great majority of cases they are accompanied by fragments of lapilli and of pumice more or less altered. Vitreous volcanic matters belonging to the acid and basic series of rocks predominate in the regions where the red clay has its greatest development; and it will be seen presently that the most characteristic decompositions which there take place are associated with pyroxenie lavas.

Associated with the red clay are almost always found concretions and microscopic particles of the oxides of iron and manganese, to which the deposit owes its colour. Again, in the typical examples of the deposit, zeolites in the form of crystals and crystalline spherules are present, along with metallic globules and silicates which are regarded as of cosmic origin. Calcareous organisms are so generally absent that they cannot be regarded as characteristic. On the other hand, the remains of Diatoms, Radiolarians, and Sponge spicules are generally present, and are sometimes very abundant. The ear-bones of various Cetaceans, as well as the remnants of other Cetacean bones and the teeth of sharks, are, in some of the typical samples far removed from the continents, exceedingly abundant, and are often deeply impregnated with, or embedded in thick coatings of, the oxides of iron and man-ganese. Over six hundred sharks' teeth, belonging to the genera Carcharodon, Oxyrhina, and Lamna, and one hundred ear-bones of whales, belonging to Ziphius, Balsenoptera, Balsena, Orca, and Delphinus, along with fifty fragments of other bones, have been obtained in one haul of the dredge in the Central Pacific. The remains of these vertebrates have seldom been dredged in the organic oozes, and still more rarely in the terrigenous deposits.

The abysmal region, in which the true pelagic deposits above described are laid down, shows a marked contrast with the " tran-sitional" or "critical area" where the terrigenous deposits are found. The former area comprises vast undulating plains from 2 to 5 miles beneath the surface of the sea, the average being about 3 miles, here and there interrupted by huge volcanic cones (the oceanic islands). No sunlight ever reaches these deep cold tracts. The range of temperature over them is not more than 7°, viz., from 31° to 38° F., and is apparently constant throughout the whole year in each locality. Plant life is absent, and, although animals belonging to all the great types are present, there is no great variety of form nor abundance of individuals. Change of any kind is exceedingly slow.

Distribution of deposits. Leaving out of view the coral and volcanic muds and sands which are found principally around oceanic islands, the blue muds, green muds and sands, red muds, together with all the coast and shore formations, are situated along the margins of the continents and in enclosed and partially enclosed seas. The chief characteristic of these deposits is the presence in them of continental debris. The blue muds are found in all the deeper parts of the regions just in-dicated, and especially near the embouchures of rivers. Red muds do not differ much from blue muds except in colour, due to the presence of ferruginous matter in greater abundance, and they are found under the same conditions as the blue muds. The green muds and sands occupy, as a rule, portions of the coast where detrital matter from rivers is not apparently accumulating at a rapid rate, viz., on such places as the Agulhas Bank, off the east coast of Australia, off the coast of Spain, and at various points along the coast of America. In the tropical and temperate zones-of the great oceans, which occupy about 110° of latitude between the two polar zones, at depths where the action of the waves is not felt, and at points to which the terrigenous materials do not extend, there are now forming vast accumulations of Globigerina and other pelagic Foraminifera, coccoliths, rhabdoliths, shells of pelagic Molluscs, and remains of other organisms. Thesedeposits may perhaps be called the sediments of median depths and of warmer zones, because they diminish in great depths and tend to disappear towards the poles. This fact is evidently in relation with the surface temperature of the ocean, and shows that pelagic Foraminifera and Molluscs live in the superficial waters of the sea, whence their dead shells fall to the bottom. Globigerina ooze is not found in enclosed seas nor in polar latitudes. In the southern hemisphere it has not been met with south of the 50th parallel. In the Atlantic it is deposited upon the bottom at a very high latitude below the warm waters of the Gulf Stream, and is not observed under the cold descending polar current which runs south in the same latitude. These facts are readily explained if it be admitted that this ooze is formed chiefly by the shells of surface organisms, which require an elevated temperature and a wide expanse of sea for their existence.

The distribution of oceanic deposits may be summarized thus. (1) The terrigenous deposits—blue muds, green muds and sands, red muds, volcanic muds and sands, coral muds and sands—are met with in those regions of the ocean nearest to land. With the exception of the volcanic muds and sands and coral muds and sands around oceanic islands, these deposits are found only lying along the borders of continents and continental islands, and. in enclosed and partially enclosed seas. (2) The organic oozes and red clay are confined to the abysmal regions of the ocean basins ; a Pteropod ooze is met with in tropical and subtropical regions in depths less than 1500 fathoms, a Globigerina ooze in the same regions between the depths of 500 and 2800 fathoms, a Radiolarian ooze in the central portions of the Pacific at depths greater than 2500 fathoms, a Diatom ooze in the Southern Ocean south of the latitude of 45° south, a red clay anywhere within the latitudes of 45° north and south at depths greater than 2200 fathoms.

As long as the conditions of the surface are the same, it might be expected that the deposits at the bottom would also remain the same. In showing that such is not the case, an agent must be taken into account which is in direct correlation with the depth. It may be regarded as established that the majority of the cal-careous organisms which make up the Globigerina and Pteropod oozes live in the surface waters, and it may also be taken for granted that there is always a specific identity between the cal-careous organisms which live at the surface and the shells of these pelagic creatures found at the bottom. Globigerina ooze is found in the tropical zone at depths which do not exceed 2400 fathoms, but when depths of 3000 fathoms are explored in this zone of the Atlantic and Pacific there is found an argillaceous deposit without, in many instances, any trace of calcareous organisms. Descending from the "submarine plateaus" to depths which exceed 2250 fathoms, the Globigerina ooze gradually disappears, passing into a greyish marl, and finally is wholly replaced by an argillaceous material which covers the bottom at all depths greater than 2900 fathoms.

The transition between the calcareous formations and the argil-laceous ones takes place by almost insensible degrees. The thinner and more delicate shells disappear first. The thicker and larger shells lose little by little the sharpness of their contour and appear to undergo a profound alteration. They assume a brownish colour, and break up in proportion as the calcareous constituent disappears. The red clay predominates more and more as the calcareous element diminishes in the deposit. Recollecting that the most important elements of the organic deposits have descended from the super-ficial waters, and that the variations in contour of the bed of the sea cannot of themselves prevent the debris of animals and plants from accumulating upon the bottom, their absence in the red clay areas can only be explained by the hypothesis of decom-position.

Pteropod ooze, it will be remembered, is a calcareous organic deposit, in which the remains of Pteropods and other pelagic Mollusca are present, though they do not always form a preponderat-ing constituent, and it has been found that their presence is in cor-relation with the bathymetrical distribution.

In studying the nature of the calcareous elements which are deposited in the abysmal areas, it has been noticed that, like the shells of the Foraminifera, those of the Thecosomatous Pteropoda, which live everywhere in the superficial waters, especially in the tropics, become fewer in number in the deposit as the depth increases. It has just been observed that the shells of Fora-minifera disappear gradually along a series of soundings from a point where the Globigerina ooze has abundance of carbonate of lime, towards deeper regions ; but it is also noticed that, when the sounding-rod brings up a graduated series of sediments from a declivity descending into deep water, among the calcareous shells those of the Pteropods and Heteropods disappear first in pro-portion as the depth increases. At depths less than 1400 fathoms in the tropics a Pteropod ooze is found with abundant remains of Heteropods and Pteropods ; deeper soundings then give a Globi-gerina ooze without these Molluscan remains ; and in still greater depths, as has been said above, there is a red clay in which cal-careous organisms are nearly, if not quite, absent.

In this manner, then, it is shown that the remains of calcareous organisms are completely eliminated in the greatest depths of the ocean. For if such be not the case, why are all these shells found at the bottom in the shallower depths, and not at all in the greater depths, although they are equally abundant on the surface at both places ? There is reason to think that this solution of calcareous shells is due to the presence of carbonic acid throughout all depths of ocean water. It is well known that this substance, dissolved in water, is an energetic solvent of calcareous matter. The investiga-tions of Buchanan and Dittmar have shown that carbonic acid exists in a free state in sea water, and Dittmar's analyses also show that deep-sea water contains more lime than surface water. This is a confirmation of the theory which regards carbonic acid as the agent concerned in the total or partial solution of the surface shells before or immediately after they reach the bottom of the ocean, and is likewise in relation with the fact that in high latitudes, where fewer calcareous organisms are found at the surface, their remains are removed at lesser depths than where these organisms are in greater abundance. It has been shown that sea water itself has some effect in the solution of carbonate of lime, and further it is probable that the immense pressure to which water is subjected in great depths may have an influence on its chemical activity. Objections have been raised to the explanation here advanced, on account of the alkalinity of sea water, but it may be remarked that alkalinity presents no difficulty which need be here considered (Dittmar, Phys. Ghem. Chall. Exp., parti., 1884).

This interpretation also explains how the remains of Diatoms and Radiolarians (surface organisms like the Foraminifera) are found in greater abundance in the red clay thair in a Globigerina ooze. The action which suffices to dissolve the calcareous matter has no effect upon the silica, and so the siliceous shells accumulate. Nor is this view of the case opposed to the distribution of the Pteropod ooze. At first it would be expected that the Foraminifera shells, being smaller, would disappear from a deposit before the Pteropod shells; but if it be remembered that the latter are very thin and delicate, and, for the quantity of carbonate of lime present, offer a larger surface to the action of the solvent than the thicker, though smaller, Globigerina shells, this apparent anomaly will be explained.

The origin of these vast deposits of clay is a problem of the highest interest. It was at first supposed that these sediments were com-posed of microscopic particles arising from the disintegration of the rocks by rivers and by the waves on the coasts. It was believed that the matters held in suspension were carried far and wide by currents, and gradually fell to the bottom of the sea. But the uni-formity of composition presented by these deposits was a great objection to this view. It can be shown that mineral particles, even of the smallest dimensions, continually set adrift upon dis-turbed waters must, owing to a property of sea water, eventually be precipitated at no great distance from land. It has also been supposed that these argillaceous deposits owe their origin to the inorganic residue of the calcareous shells which are dissolved away in deep water, but this view has no foundation in fact. Everything seems to show that the formation of the clay is due to the decomposition of fragmentary volcanic products, whose presence can be detected over the whole floor of the ocean.

These volcanic materials are derived from floating pumice, and from volcanic ashes ejected to great distances by terrestrial volcanoes, and carried far by the winds. It is also known that beds of lava and of tufa are laid down upon the bottom of the sea. This assemblage of pyrogenic rocks, rich in silicates of alumina, decomposes under the chemical action of the water, and gives rise, in the same way as do terrestrial volcanic rocks, to argillaceous matters, according to re-actions which can always be observed on the surface of the globe, and which are too well known to need special mention here.

The universal distribution of pumice over the floor of the ocean is very remarkable, and would at first appear unaccountable; but when the fact that pieces of pumice have been known to float in sea water for a period of over three years before becoming suffici-ently waterlogged to sink is taken into consideration, it will be readily understood how fragments of this material may be trans-ported by winds and currents to an enormous distance from their point of origin before being deposited upon the bottom. Fragments of pumice are dredged in the greatest profusion in the red clay of the Central Pacific, and much less abundantly in the or-ganic oozes and terrigenous deposits. This is owing to the rate of deposition being much slower in the former than in the latter, where the rapid accumulation of calcareous and siliceous organisms and continental debris masks their presence.

The detailed microscopic examination of hundreds of soundings has shown that the presence of pumice, of lapilli, of silicates, and of other volcanic minerals in various stages of decomposition can always be demonstrated in the argillaceous matter.

In the places where the red clay attains its most typical develop-ment, the transformation of the volcanic fragments into argillaceous matter may bo followed step by step. It may be said to be the direct product of the decomposition of the basic rocks, represented by volcanic glasses, such as hyalomelan and tachylite. This decom-position, in spite of the temperature approximating to zero (32° F.), gives rise, as an ultimate product, to clearly crystallized minerals, which may be considered the most remarkable products of the chemical action of the sea upon the volcanic matters undergoing decomposition. These microscopic crystals are zeolites lying free in the deposit, and are met with in greatest abundance in the typical red-clay areas of the Central Pacific. They are simple, twinned, or spheroidal groups, which scarcely exceed half a millimetre in diameter. The crystallographic and chemical study of them shows that they must be referred to christianite. It is known how easily the zeolites crystallize in the pores of eruptive rocks in process of (fig. 5), have been formed at the expense of the decomposing volcanic matters spread out upon the bed of that ocean.

In connexion with this formation of zeolites, reference may be made to a chemical process which gives rise to the formation of nodules of manganiferous iron. These nodules are almost uni-versally distributed in oceanic sediments, but are met with in the greatest abundance in the red clay. This association tends to show a common origin. It is exactly in those regions where there is an accumulation of pyroxenic lavas in decomposition, containing sili-cates with a base of manganese and iron, such for example as augite, hornblende, olivine, magnetite, and basic glasses, that manganese nodules occur in greatest numbers. In the regions where the sedimentary action, mechanical and organic, is, as it were, suspended, and where everything shows an extreme slowness of deposition,—in these calm waters favourable to chemical reac-tions, ferro - manganiferous substances form concretions around organic and inorganic centres.

These concentrations of ferric and manganic oxides, mixed with argillaceous materials whose form and dimensions are extremely variable, belong generally to the earthy variety or wad, but pass sometimes, though rarely, into varieties of hydrated oxide of manganese with distinct indications of radially fibrous crystalliza-tion.^ The interpretation necessary, in order to explain this formation of manganese nodules, is the same as that admitted in explanation of the formation of coatings of this material on the surface of terrestrial rocks. These salts of manganese and iron, dissolved in water by carbonic acid, then precipitated in the form of carbonate of protoxide of iron and manganese, become oxidized, and give rise in the calm and deep oceanic regions to more or less pure ferro-manganiferous concretions. At the same time it must be admitted that rivers may bring to the ocean a contribution of the same substances.

Among the bodies which, in certain regions where red clay predominates, serve as centres for these mangani-ferous nodules are the re-mains of vertebrates. These remains are the hardest parts of the skeleton—tympanic bones of whales, beaks of Ziphius, teeth of sharks ; and, just as the calcareous

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FIG.6.—SectionofaManganeseNodu)e,enclos-shclls are eliminated in ing tympanic hone of a whale, from 2300 great depths, so all the re- fathoms, South Pacific, mains of the larger vertebrates are absent, except the most resistant portions.

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These bones often serve as a centre for the manganese iron concretions, being frequently surrounded by layers several centimetres in thickness (fig. 6). In the same dredgings in the red-clay areas some sharks' teeth and Cetacean ear-bones, some of which belong to extinct species, are surrounded with thick layers of the manganese, and others with merely a slight coating. Cosmic The cosmic spherules incidentally referred to under the descrip-sphe- tion of red clay may be here described in greater detail. If a rules, magnet be plunged into an oceanic deposit, especially a red clay from the central parts of the Pacific, particles are extracted, some of which are magnetite from volcanic rocks, to which vitreous matters are often attached ; others again are quite isolated, and differ in most of their properties from the former. The latter are generally round, measuring hardly 0'2 mm., usually smaller; their surface is quite covered with a brilliant black coating having all the properties of magnetic oxide of iron; often there may be noticed clearly marked upon them cup-like depressions (figs. 7 and 8). If these spherules be broken down in an agate mortar, the brilliant black coating easily falls away and reveals white or grey metallic malleable nuclei, which may be beaten out by the pestle into thin lamellae. This metallic centre, when treated with an acid solu-tion of sulphate of copper, immediately assumes a coppery coat, thus showing that it is native iron. But there are some mal-leable metallic nuclei extracted from the spherules which do not give this reaction; they do not take the copper coating.

Fig. 7. Fig. 8.

FIG. 7.—Black Spherule with Metallic Nucleus (xGO). This spherule covered with a coating of black shining magnetite represents the most frequent shape. The depression here shown is often found at the surface of these spherules. From 2375 fathoms, South Pacific.

FIG. 8.—Black Spherule with Metallic Nucleus (x60). The black external coat-ing of magnetic oxide has been broken away to show the metallic nucleus re-presented by the clear part at the centre. From 3150 fathoms.

Chemical reactions show that they contain cobalt and nickel; very probably they constitute an alloy of iron and these two metals, such as is often found in meteorites, and whose presence in large quantities hinders the production of the coppery coating on the iron. G. Rose has shown that this coating of black oxide of iron is found on the periphery of meteorites of native iron, and its presence is readily understood when their cosmic origin is admitted. Indeed, these meteoric particles of native iron in their transit through the air must undergo combustion, and, like small portions of iron from a smith's anvil, be transformed either entirely or at the surface only into magnetic oxide, and in the latter case the nucleus is protected from further oxidation by the coating which thus covers it.

One may suppose that meteorites in their passage through the atmosphere break into numerous fragments, that incandescent particles of iron are thrown off all round them, and that these eventually fall to the surface of the globe as almost impalpable dust, in the form of magnetic oxide of iron more or less completely fused. The luminous train of falling stars is probably due to the combustion of these innumerable particles resembling the sparks which fly from a ribbon of iron burnt in oxygen, or the particles of the same metal thrown off when striking a flint. It is easy to show that these particles in burning take a spherical form, and are surrounded by a layer of black magnetic oxide.

Among the magnetic grains found under the same conditions as those just described are other spherules, which are referred to the chondres, so that, if the interpretation of a cosmic origin for the magnetic spherules with a metallic centre were not established in a manner absolutely beyond question, it almost becomes so when their association with the silicate spherules, which will now be described, is taken into account. It will be seen by the microscopic details that these spherules have quite the constitution and structure of chondres so frequent in meteorites of the most ordinary type, and on the other hand they have never been found, as far as is known, in rocks of a terrestrial origin ; in short, the presence of these spherules in the deep-sea deposits, and their association with the metallic spherules, are matters of prime importance.

Among the fragments attracted by the magnet in deep-sea deposits are distinguished granules slightly larger than the spherules with the shining black coating above described. These are yellowish-brown, with a bronze-like lustre, and under the microscope it is noticed that the surface, instead of being quite smooth, is grooved by thin lamella;. Their dimensions never attain a millimetre, generally they are about 0 '5 mm. in diameter ; they are never perfect spheres, as in the ease of the black spherules with a metallic centre; and sometimes a depression more or less marked is to be observed in the periphery. When examined by the micro-scope it is observed that the lamella? which compose them are applied the one against the other, and have a radial eccentric dis-position. It is the leafy radial structure (radialblattrig), like that of the chondres of bronzite, which predominates in these spherules. The serial structure of the chondres with olivine is observed much less rarely, and indeed there is some doubt about the indications of this last type of structure. Fig. 9 shows the characters and texture of one of these spherules magnified 25 diameters. On account of their small dimensions, as well as of their friability due to their lamellar structure, it is difficult to polish one of these spherules, and it has been necessary to study them with reflected light, or to limit the observations to the study of the broken fragments.

These spherules break up following the lamella?, which latter are seen to be extremely fine and perfectly transparent. In rotating between crossed nicols they have the extinctions of the rhombic system, and in making use of the condenser it is seen that they have one optic axis. It is observed also that when several of these lamella? are attached they extinguish exactly at I the same time, so that I everything tends to show that they form a single individual.

In studying these trans-parent and very thin frag-ments with the aid of a high magnifying power, it is ob-served that they are dotted with brown-black inclu-sions, disposed with a cer-tain symmetry, and showing somewhat regular contours; these inclusions are referred to magnetic iron, and their presence explains why these spherules of bronzite are extracted by the magnet. It should be observed, however, that they are not so strongly magnetic as those with a metallic nucleus.

They are designated bronzite rather than enstatite, because of the somewhat deep tint which they present; they are insoluble in hydrochloric acid. Owing to the small quantity of substance, only a qualitative analysis could be made, which showed the presence in them of silica, magnesia, and iron.

The study of deep-sea deposits suggests some interesting conclu- Great sions. It has been said that the debris carried away from the land anti-accumulates at the bottom of the sea before reaching the abysmal quity of regions of the ocean. It is only in exceptional cases that the finest oceanic terrigenous materials are transported several hundred miles from areas, the shores. In place of layers formed of pebbles and clastic elements with grains of considerable dimensions, which play so large a part in the composition of emerged lands, the great areas of the ocean basins are covered by the microscopic remains of pelagic organisms, or by the deposits coming from the alteration of volcanic products. The distinctive elements that appear in the river and coast sediments are, properly speaking, wanting in the great depths far distant from the coasts. To such a degree is this the case that in a great number of soundings, from the centre of the Pacific for example, no mineral particles on which the mechanical action of water had left its imprint have been distinguished, and quartz is so rare that it may be said to be absent. It is sufficient to indicate these facts in order to make apparent the profound differences which separate the deposits of the abysmal areas of the ocean basins from the series of rocks in the geological formations. As regards the vast deposits of red clay, with its manganese concretions, its zeolites, cosmic dust, and remains of vertebrates, and the organic oozes which are spread out over the bed of the Central Pacific, Atlantic, and Indian Oceans, have they their analogues in the geological series of rocks? If it be proved that in the sedimentary strata the true pelagic sediments are not represented, it follows that deep and extended oceans like those of the present day cannot formerly have occupied the areas of the present continents, and as a corollary the great lines of the oceanic basins and continents must have been marked out from the earliest geological ages.

Without asserting that the terrestrial areas and the areas covered by the waters of the great ocean basins have had their main lines marked out since the commencement of geological history, it is a fact proved by the evidence of the pelagic sediments that these areas have a great antiquity. The accumulation of sharks' teeth, of the ear-bones of Cetaceans, of manganese concretions, of zeolites, of volcanic material in an advanced state of decomposition, and of cosmic dust, at points far removed f~om the continents, tends to prove this. There is no reason for supposing that the parts of the ocean where these vertebrate remains are found are more frequented by sharks or Cetaceans than other regions where they are never, or only rarely, dredged from the deposits at the bottom. When it is remembered also that these ear-bones, teeth of sharks, and volcanic fragments are sometimes incrusted with two centimetres of manganese oxide, while others have a mere cqating, and that some of the bones and teeth belong to extinct species, it may be concluded with great certainty that the clays of these oceanic basins have accumulated . with extreme slowness. It is indeed almost beyond question that the red-clay regions of the Central Pacific contain accumulations belonging to geological ages different from our own. The great antiquity of these formations is likewise confirmed in a striking manner by the presence of cosmic fragments, the nature of which has been described. In order to account for the accumulation of all these substances in such relatively great abundance in the areas where they were dredged, it is necessary to suppose the oceanic basins to have remained the same for a vast period of time.

The sharks' teeth, ear-bones, manganese nodules, altered volcanic fragments, zeolites, and cosmic dust are met with in greatest abundance in the red clays of the Central Pacific, at that point on the earth's surface farthest removed from continental land. They are less abundant in the Radiolarian ooze, are rare in the Globi-gerina, Diatom, and Pteropod oozes, and have been dredged only in a few instances in the terrigenous deposits close to the shore. These substances are present in all the deposits, but owing to the abundance of other matters in the more rapidly forming deposits their presence is masked, and the chance of dredging them is reduced. The greater or less abundance of these materials, which are so characteristic of a true red clay, may be regarded as a measure of the relative rate of accumulation of the marine sediments in which they lie. The terrigenous deposits accumulate most rapidly ; then follow in order Pteropod ooze, Globigerina ooze, Diatom ooze, Radiolarian ooze, and, slowest of all, red clay. Geologi- From the data now advanced it appears possible to deduce other cal as- conclusions important from a geological point of view. In the pects of deposits due essentially to the action of the ocean, the great variety deposits, of sediments which may accumulate in regions where the external conditions are almost identical is very striking. Again, marine faunas and floras, at least those of the surface, differ greatly, both with respect to species and the relative abundance of individuals, in different regions of the ocean; and, as their remains determine the character of the deposit in many instances, it is legitimate to conclude that the occurrence of organisms of a different nature in several beds is not an argument against the synchronism of the layers which contain them. In this connexion may be noted the fact that in certain regions of the deep sea no appreciable forma-tion is now taking place. Hence the absence, in the sedimentary series, of a layer representing a definite horizon must not always be interpreted as proof either of the emergence of the bottom of the sea during the corresponding period, or of an ulterior erosion.

The small extent occupied by littoral formations, especially those of an arenaceous nature, and the relatively slow rate at which such deposits are formed along a stable coast, are matters of importance. In the present state of things there does not appear to be any-thing to account for the enormous thickness of the clastic sediments making up certain geological formations, unless the exceptional cases of erosion which are brought into play when a coast is under-going constant elevation or subsidence are considered. Great move-ments of the land are doubtless necessary for the formation of thick beds of transported matter like sandstones and conglomerates. Arenaceous formations of great thickness require seas of no great extent and coasts subject to frequent oscillations, which permit the shores to advance and retire. Along these, through all periods of the earth's history, the great marine sedimentary phenomena have taken place.

The continental geological formations, when compared with marine deposits of modern seas and oceans, present no analogues to the red clays, Radiolarian, Globigerina, Pteropod, and Diatom oozes. On the other hand, the terrigenous deposits of lakes, shallow seas, enclosed seas, and the shores of the continents reveal the equivalents of the chalks, greensands, sandstones, conglomerates, shales, marls, and other sedimentary formations. Such formations as certain Tertiary deposits of Italy and the Radiolarian earth from Barbados, where pelagic conditions are indicated, must bo regarded as having been laid down rather along the border of a continent than in a true oceanic area. The white chalk is evidently not a deep-sea deposit, for the Foraminifera and fragments of other organisms of which it is largely composed are similar to those found in comparatively shallow water not far from land. The argillaceous and calcareous rocks recently discovered by Dr Guppy in the upraised coral islands in the Solomon group are identical with the deposits now forming around oceanic islands. Regions situated similarly to enclosed and shallow seas and the borders of the present continents appear to have been, throughout all geological ages, the theatre of the greatest and most remarkable changes ; in short, all, or nearly all, the sedi-mentary rocks of the continents would seem to have been built up in areas like those now occupied by the terrigenous deposits.

During each era of the earth's history the borders of some lands have sunk beneath the sea and been covered by marine sediments, while in other parts the terrigenous deposits have been elevated into dry land, and have carried with them a record of the organisms which flourished in the sea of the time. In this transitional area there has been throughout a continuity of geological and biological phenomena.

From these considerations it will be evident that the character of a deposit is determined much more by distance from the shore of a continent than by actual depth ; and the same would appear to be the case with respect to the fauna spread over the floor of the present oceans. Dredgings near the shores of continents, in depths of 1000, 2000, or 3000 fathoms, are more productive both in species and in-dividuals than dredgings at similar depths several hundred miles seawards. Again, among the few species dredged in the abysmal areas farthest removed from land, the majority show archaic characters, or belong to groups which have a wide distribution in time as well as over the floor of the present oceans. Such are the Hexactinellida, BracMopoda, Stalked Crinoids and other Echino-derms, &c.

As already mentioned, the "transitional area" is that which now shows the greatest variety in respect to biological and physical conditions, and in past time it has been subject to the most frequent and the greatest amount of change. The animals now living in this area may bo regarded as the greatly modified descendants of those which have lived in similar regions in past geological ages, and some of whose ancestors have been preserved in the sedimentary rocks as fossils. On the other hand, many of the animals dredged in the abysmal regions are most probably also the descendants of animals which lived in the shallower waters of former geological periods, but migrated into deep water to escape the severe struggle for existence which must always have obtained in shallower waters influenced by light, heat, motion, and other favourable conditions. Having found existence possible in the less favourable and deeper water, they may be regarded as having slowly spread themselves over the floor of the ocean, but without undergoing great modifications, owing to the extreme uniformity of the conditions and the absence of competition. Or it may be supposed that, in the depressions which have taken place near coasts, some species have been gradually carried down to deep water, have accommodated themselves to the new conditions, and have gradually migrated to the regions far from land. A few species may thus have migrated to the deep sea during each geological period. In this way the origin and distribution of the deep-sea fauna in the present oceans may in some measure be explained. In like manner, the pelagic fauna and flora of the ocean is most probably derived originally from the shore and shallow water. During each period of the earth's history a few animals and plants have been carried to sea, and have ultimately adopted a pelagic mode of life.

ISLANDS.

The Pacific Ocean is distinguished from the Atlantic by the greater number of small island groups that diversify its surface. The continental islands, lying along the coasts of America and Asia, have been referred to in speaking of the coasts; the islands of the Malay Archipelago, Australia, New Zealand, and probably New Caledonia belong to the same class. The true oceanic islands on the other hand have no direct geological connexion with the continents, the older sedimentary and metamorphic rocks appear to bo quite absent, the islands being either of eruptive or coral formation. The fauna and flora of the oceanic islands present a considerable amount of uniformity, though each island or important group of islands has its peculiar species. There is an entire absence of terrestrial Mammalia. The genera and species are few in number when compared with those of the continents and continental islands from which they would appear to have been originally derived by immigration, and subsequently to have undergone modifica-tion. Recent researches appear also to show that the dredgings around oceanic islands yield fewer genera and species than dredgings at similar depths along the shores of continents, although the numbers of individuals of a few species may be extraordinarily abundant.

The most northern oceanic group is the Hawaiian Archipelago or Sandwich Islands (see vol. xi. p. 528), stretching for about 340 miles between the latitudes of 18° 52' and 22° 15' N, and the meridians of 154° 42' and 160° 33' W.; it consists of eight large islands—Hawaii (Owhyhee), Maui (Mowee), Kahulaui (Tahooroway), Lanai (Ranai), Molokai (Morotoi), Oahu (Woahoo), Kauai (Atooi), and Niihau (Oneehoow), and four small barren islets, the entire area being 6100 square miles. The islands of this group are mountainous, and abound in active volcanoes; the great lake of fire, Kilauea, on the east side of the Mountain of Mauna Loa (13,760 feet) in Hawaii is probably the largest active crater in the world, while one of the largest known extinct craters is that of Mauna Haleakala ("The House of the Sun") in Maui, at a height of 10,200 feet above the sea; it is 12 miles in circumference. The Hawaiian Islands being within the zone of coral formation are surrounded by fringing reefs, and there is abundant evidence that gradual upheaval has taken place over the whole area which they occupy. There are beds of coral limestone in Molokai at a height of 400 feet, and in Kauai coral sand is found at an elevation of 4000 feet above the sea; in many other islands coral and lava are found interstratified.

The three groups of the Bonin Islands known as the Parry, Beechy, and Coffin groups are composed of high rocky islets of a bold and fantastic outline, and are situated between 26° and 27° N. lat.

The Ladrones or Mariana Islands (see vol. xiv. p. 199) have a total area of 395 square miles; they stretch for nearly 450 miles between 13° and 20° N. lat. and 144° 37' and 145° 55' E. long. These islands are all of volcanic origin, and their mountains contain several active volcanoes.

The Caroline Archipelago (see vol. v. p. 125) lies about 170 miles to the south of the Ladrones, and, together with the Pelew Islands, has an area of 877 square miles. The Carolines embrace forty distinct island groups, five of which are basaltic and mountainous, though surrounded by coral reefs ; the remaining thirty-five groups are entirely of coral formation, and do not rise much above the sea-level. The Pelew Islands resemble the Carolines in their physical characters ; they present peculiarities in the arrangement of atolls which will be alluded to below.

The Marshall Islands (see MICRONESIA, vol. xvi. p. 256) consist of two chains running parallel to each other, and composed of fourteen and seventeen small groups respec-tively. They lie to the eastward of the Carolines, and are entirely of organic formation.
The Gilbert Archipelago (see vol. xvi. p. 256) is cut by the equator. It contains sixteen groups of small coral islands, low and barren, but densely populated.

In the South Pacific oceanic islands are scattered with the greatest profusion over a region between 5° and 25° S. lat. and 180° to 120° W. long. The northern part of the shallow water surrounding Australia, New Zealand, and the Malay Archipelago is occupied by the Solomon Islands, the New Hebrides, the bold rocky and mountain-ous islands of Fiji with fine barrier reefs, the Friendly Islands, and Samoa or the Navigators' Islands. Farther to the south there are the Society Islands, including Tahiti; they are lofty, of volcanic origin, and surrounded by very perfect barrier reefs. The Marquesas or Mendana Archi-pelago, farther to the north, also consists of volcanic islands, but they are not fringed by reefs.

The volcanic group of the Galapagos Archipelago is situated under the equator at a distance of 500 or 600 miles from the west coast of South America; it has been minutely described by Darwin.

The extensive Low or Paumotu Archipelago lies to the south-east of the Society Islands, and runs, parallel to them. It consists of about eighty atolls, some of them of large size, and all typical examples of this form of coral island.

The total area of the islands of the Pacific is exceedingly small, especially when the vast number of groups that stud the ocean is taken into consideration.

THEORY OF CORAL ISLANDS.

The origin of coral islands was specially studied by Darwin during the voyage of the " Beagle " in 1831-36, and he shortly afterwards published a theory on the subject which has been fully detailed in the article CORAL (vol. vi. p. 377). This theory was so simple, and it appeared so complete,, that it acquired universal acceptance; and the continuous action of subsidence in promoting the development of fringing reefs into barriers, and of barriers into atolls, was long unquestioned. In 1851 L. Agassiz expressed the opinion that the theory of subsidence was insufficient to explain the formation of the coral reefs and keys of Florida. In 1863 Carl Semper stated that an attentive study of the Pelew Islands showed the complete inadequacy of this theory, and in 1868 he reiterated his convictions.

In 1880 Mr John Murray published an abstract of his "Chal-lenger" observations, and gave a theory of coral island formation which claims to account for all the phenomena without calling in the aid of subsidence. It is pointed out that, with hardly an exception, the oceanic islands are of volcanic origin, and it is assumed that the various peaks which deep-sea soundings have shown to be scattered over the bed of the ocean, and rising to within various distances of the surface, are also, primarily, of volcanic origin. There is no evidence of any extensive submerged continent or mass of land such as Darwin's theory requires. Whether built up sufficiently high to rise above the surface of the sea and thus form islands, or brought up only to varying heights below the sea-level, these volcanic eminences tend to become platforms on which coral reefs may be formed. The erosive action of waves and tides tends to reduce all volcanic summits down to the lower limit of breaker action, thus producing platforms on which barrier reefs and atolls may spring up. Again, submarine eminences may be brought up to the zone of the reef builders by the deposit of volcanic and organic detritus falling from the surface, as well as through the agency of organisms secreting lime and silica, which live in profusion at great depths, especially on the tops of submarine peaks and banks. The great profusion of life in the tropical surface waters is insisted upon, and it is pointed out that this pelagic life supplies the reef-building corals with food, and that, when these surface creatures die and their shells fall to the bottom, they carry down with them sufficient organic matter to furnish food to the animals living on the floor of the ocean. As the result of tow-net experiments in the tropics Mr Murray estimated that, in the surface waters of the ocean, there were in a mass 1 mile square by 100 fathoms, 16 tons of carbonate of lime existing in the form of shells of pelagic Foraminifera and Molluscs. In this way it is urged that submarine banks are continually being brought within the zone of reef-building corals. Darwin admitted that reefs not to be distinguished from atolls might be formed on such submarine banks, but the improbability of so many submerged banks existing caused him to dismiss this explanation without further consideration. He was not, however, aware of the great number of submerged cones which recent soundings have made known, nor of the enormous abundance of minute calcareous organisms—such as calcareous algte, Foraminifera, and Molluscs in the surface waters—and of the comparatively rapid rate at which their remains might accumulate on the sea bottom. Nor had he any idea of the comparatively great abundance of animals living at considerable depths.

Coral-reef builders starting on a bank, whether formed by elevation or subsidence, by erosion or the upward growth of deep-sea deposits composed largely of organic remains, tend ultimately to assume the atoll or barrier form. When the coral reef or colony approaches the surface, the central portions are gradually placed at a disadvantage as compared with the peripheral parts of the mass, in being farther removed from the food supply which is brought by the oceanic currents, and consequently dwindle and die. In pro-portion as the reef approaches the surface, the centre becomes cut off from the food supply and the conditions become increasingly uncongenial. At last an outer ring of vigorously growing reef corals encloses a central lagoon. The windward side of the reef grows most vigorously, not because of a larger supply of oxygen and greater aeration of the water, but because that is the direction in which the oceanic currents bring the food to the reef. As the atoll extends seawards from vigorous growth the lagoon becomes larger, chiefly from the removal of lime in solution by the action of the carbonic acid in sea water which flows in and out at each tide. This solvent action of sea water on dead calcareous organ-isms was shown by the " Challenger's " observations to be uni-versal.

Mr Murray reverses the order of growth as given by Darwin for the groups in the Indian Ocean. He regards the Laccadive, Caro-line, and Chagos archipelagos as various stages in the growth of coral reefs towards the surface, and he explains the various appearances in the Maldive group of atolls without any necessity for disseverment by oceanic currents as argued by Darwin. Precisely the same explanation is applied to the ease of a barrier reef. It commences in the shallow water near the shore, and afterwards extends seawards on a talus built up of lumps of coral broken off by the surf. A very careful examination of the barrier reef at Tahiti was made by Lieutenant Swire of H.M.S. "Challenger" and Mr Murray, and they found that such an explanation was completely justified by the form and nature of the reef. There was much dead coral on the inner side of the barrier, which in many places was perpendicular or even overhanging; while, on the contrary, the outer surface was all alive, and sloped gradually sea-wards. A section of it, drawn to a true scale, is given in fig. 10.

FIG. 10.—Section across the Barrier Reef, Tahiti.

This section shows that a ledge, over which there is a depth of from 30 to 40 fathoms of water, runs out for 250 yards from the edge of the reef. This ledge is covered with luxuriant heads and bosses of coral. Beyond it there is a steep irregular slope at an angle of about 45°, the talus being formed apparently of coral masses broken off from the ledge, and piled up; this slope is covered with living Sponges, Alcyonarians, Hydroids, Polyzoa, Foraminifera, and other forms of life. The angle of inclination then decreases to 30°, and the ground is covered with coral sand; while beyond 500 yards from the edge of the reef the declivity is insignificant, only 6°, and there is a bed of mud containing volcanic and coral sand mixed with Pteropod and other shells, in 590 fathoms of water. The vast perpendicular wall of coral limestone descending into unfathomable depths, which has been supposed usually to mark the outside of a coral reef, has always been looked upon as a conclusive proof of great subsidence having taken place; but the depth and the slope of such limestone walls have been greatly exaggerated, and no means have been taken to ascertain beyond doubt that the rock is formed of coral throughout. The probability is that only the upper portion of such a wall is true coral limestone; and Dr Guppy has recently shown that this is actually the case in some upraised coral islands of the Solomon group. Upheaval has taken place to a considerable extent in the oceanic islands, and more extended examination of the limestone cliffs of other coral islands will probably lead to the discovery of many such cases. Mr Murray holds that the characteristic form of barrier reefs and atolls is in no way dependent on subsidence, that subsidence is not the cause of their peculiar features, that these reefs may be met with indifferently in stationary areas, in areas of subsidence, and in areas of elevation, and that elevation and sub-sidence only modify in a minor way the appearance of the islands.

The chief phenomena are accounted for—(1) by a physiological fact,—the very vigorous growth of the reef-forming species on the outer or seaward face of the reef where there is abundance of food, and the much less vigorous growth, and even death, of these species on the inner parts of the reefs and in the lagoons, where there is much less food, and where there are other conditions inimical to growth; and (2) by a physical and chemical fact,—the removal of lime in suspension and in solution from the inner portions of the reefs and from the lagoons, where much dead coral is exposed to the action of sea water containing carbonic acid, the result being the formation, the deepening, and the widening of lagoons and lagoon channels.

For further information on subjects referred to in this article see John Murray, " On the Structure and Origin of Coral Reefs and Islands," Proc. Roy. Soc. Edin., vol. x. p. 505; Alex. Agassiz, "On the Tortugas and Florida Reefs," Trans. Amer. Acad., vol. xi. (1885); Archd. Gelkie, "The Origin of Coral Reefs," Nature, vol. xxix. pp. 107 and 124; John Murray and A. Renard, "On the Nomenclature, Origin, and Distribution of Deep-Sea Deposits," Proc. Roy. Soc. Edin,, vol. xii. p. 495 (1884); John Murray and A. Renard, "On the Microscopic Characters of Volcanic Ashes and Cosmic Dust, and their Distribution in the ' Deep-Sea Deposits," Proc. Roy. Soc, Edin., vol. xii. p. 474, 1884. (J. MU.)


Footnotes

1 For the composition of these manganese nodules, see MANGANESE, vol. xv. p. 479.



The above article was written by: John Murray, Director of the Challenger Expedition Commission.



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