1902 Encyclopedia > Granite

Granite




GRANITE, a rock so named from the Latin granum, a grain, in allusion to the granular texture of many of its varieties. The term appears to have been introduced by the early Italian antiquaries, and it is believed that the first recorded use of the word occurs in a description of Rome by Flaminius Vacca, an Italian sculptor of the 16th century. This description was published by Moutfaucon in his Diarium Italicum, where we read of certain columns " ex marmore granito iEgyptio " (cap. xvii), and of others "ex marmore granito .¿Ethalise instüae" (cap. xviii.), showing that the Romans of Vacca's day were acquainted with granite from Egypt and from Elba. Granite is also referred to by Caesalpinus in his treatise De Metallicis (1596), and by Tournefort in his Relation d'un Voyage au Levant (1698); indeed the latter has been cited by Emmerling (Lehrb. d. Mineral.) as the first author who uses the term. By these early writers, however, the name was loosely applied to several distinct kinds of granular rock, and it remained for Werner to give it that precise meaning which it at present possesses as the specific designation of a rock.

Granite is a crystalline-granular rock consisting, in its typical varieties, of orthoclase, quartz, and mica, to which a plagioclastic felspar is usually added. These minerals are aggregated together without the presence of any matrix or connecting medium. Thin sections of a true granite, examined under the microscope by transmitted light, show no trace of any amorphous or crypto-crystalline ground-mass. The chemical composition of the rock will, of course, vary with its mineralogical constitution. For an average analysis see GEOLOGY, vol. x. p. 233. The proportion of silica varies from 62 to 81 per cent. Granite belongs therefore to Buusen's class of acid rocks, or those which contain more than 60 per cent, of silica. Dr Haughton has found an exceptionally low proportion of this oxide in some of the Irish granites (58'44 per cent., e.g., in some Donegal granite, Quart. Journ. Geol. Soc, xviii., 1862, p. 408). The specific gravity of granite varies from 2'59 to 2'73.

Orthoclase, or potash felspar, is the principal constituent of most granites. This mineral occurs either in simple crystals, or in twins formed on what is known as the " Carlsbad type," such crystals being common at Carlsbad in Bohemia. In porphyritic granites, such as those of Cornwall and Devon, the orthoclase crystals may attain to a length of several inches, and the twinning is marked on the fractured crystals by a line running longitudinally down the middle, and dividing the crystal into two halves. In colour the orthoclase generally varies between snow-white and flesh-red. The green felspar known as Amazon stone, which occurs in certain granites, has lately been shown by Des Cloiseaux to belong to the species microcline, and not, as previously supposed, to orthoclase (Annates d. Gh., 5 ser., ix., 1876, p. 433). The plagioclastic, anorthic, or triclinic felspar of granite occurs in crystals which are generally smaller than those of the orthoclase, and which exhibit, even to the naked eye, their characteristic twin striation. Moreover the lustre is frequently resinous or fatty, while that of the orthoclase is pearly on the cleavage-planes. In most cases the plagioclase is the soda-lime felspar called oligoclase; but in some granites it is cdbite or soda felspar, as shown by Haughton in many of the Irish and Cornish granites (Proc. Roy. Soc, xvii., 1869, p. 209). When a granite becomes weathered, the felspar may decompose into kaolin or china-clay ; the commencement of this alteration is indicated under the microscope by the turbidity of the felspar, by the ill-defined edges of the crystals, and in the case of plagioclase by disappearance of the characteristic striae.





The quartz of granite occurs generally in irregularly-shaped angular grains ; but occasionally in distinct crystals which are double hexagonal pyramids with or without the corresponding prism. Colourless quartz is most common, but grey, brown, or bluish varieties also occur. Whatever its colour, it is as a rule transparent in microscopic sections, though sometimes rendered milky by the presence of vast numbers of minute cavities containing liquid (see GEOLOGY, ut supra, and for Sorby's original researches Quart. Journ. Geol. Soc,, xiv. p. 453). In many granites the quartz fills up the spaces between the crystals of felspar and of mica, and receives impressions from these minerals. This fact has been advanced against the view that granite has existed in a state of fusion; since it is assumed that, as the quartz is the most infusible of the three component minerals, it would have been the first to solidify on the cooling of the magma, whereas the relation of the quartz to the associated minerals in most cases shows that it must have solidified after the crystallization of the felspar and mica. In some granites, however, the quartz is developed in free crystals, thus pointing to an early solidification of this mineral. The mica, which is usually the least abundant constituent of the granite, occurs in thin scales of irregular shape or in hexagonal plates. It is either a white biaxial potash mica (muscovite) or a dark-brown magnesian mica, generally uniaxial (biotite). Both species may occur in the same granite. Haughton has shown that some of the white mica of the Cornish granites is lepidolite, or lithia mica; while some of the black mica in the same rocks is the iron-potash mica, lepidomelane (Proc. Roy. Soc, xvii. p. 209). | Professor Heddle finds that the black mica of most Scottish granites is a distinct species, which he calls HaugMonile (Mineralog. Mag., No. 13,1879, p. 72). A large number of accessory minerals occur in granite, no fewer than forty-four being cited by Zirkel (Lehrb. d. Petrog., i. p. 481). Upon the presence of these supplementary minerals numerous varieties of granite have been founded. Thus, if tourmaline be present, the rock is a schorlaceous or tourmaline granite; when cassiterite or tin-stone occurs, it forms a stanniferous granite; the presence of epidote gives rise to an epidote granite ; and so on with other minerals. The most common accessory constituent of granite is hornblende, a mineral which appears to replace to some extent the mica, and thus produces a hornblendic or syenitic granite. This rock was formerly, and by some petrographers is still, termed syenite ; it is the syenites of Pliny, so named from Syene in Upper Egypt, where a smiliar rock was quarried by the ancient Egyptians. By modern petrographers, however, the term syenite is usually restricted to a rock which is an aggregate of orthoclase and hornblende,—in other words, a granite in which the quartz has disappeared while the mica has been superseded by hornblende. A beautiful schorla-ceous rock, which is apparently a variety of granite, has been described by Pisani under the name of luxullianite (Gom-ptes Rendus, lix., 1864, p. 913). It occurs in the parish of Luxullian, near Lostwithiel, in Cornwall, where it is found in the form of boulders, but has not been detected in \ situ. This rock is composed of tourmaline, or schorl, with i quartz and orthoclase ; the last named mineral occurring" in j large flesh-coloured crystals, which by contrast with the | dark base produce a very beautiful effect. Two varieties of tourmaline, one brown and the other bluish, have been detected by Professor Bonney (Mineralog. Mag., No. 7, 1877, p. 215). The sarcophagus of the duke of Wellington, in St Paul's Cathedral, is wrought out of a splendid block of luxullianite. Many varieties of granite are founded upon structural characteristics. Occasionally the constitu-ents are developed in such large individuals as to form a giant granite. Crystals of orthoclase, associated with quartz in a peculiar parallel arrangement, produce the variety known as graphic granite or Lapis Judaicus—names which refer to the resemblance which the rock presents, when cut in certain directions, to lines of Hebrew charac-ters. Graphic granite was termed by Hauypegmatite, but this name is now generally applied to a coarse admixture i of orthoclase, quartz, and silvery mica. When any of the component minerals occur in large crystals, embedded in a i fine-grained base, a porphyritic granite is produced. Gene-rally the crystals are those of orthoclase, as in many of the West of England granites, and in the characteristic rock of Shap Fell in Westmoreland. Granitite is a name applied \ to a variety of granite made up of orthoclase and quartz, with more or less plagioclase and a small proportion of mica. A granite composed of only felspar and quartz is called haplite or semi-granite. Some of the micaless varieties are known as granidite. When, instead of the mica disappear-ing, the felspar is absent, the resulting aggregate of quartz and mica is termed greisen; it is frequently a tin-bearing \ rock. Occasionally the granite, when fine in grain, loses j its mica., and an intimate mixture of orthoclase and quartz is thus obtained; such a rock is known as a felstone. Crystals of orthoclase disseminated through a felsitic matrix, either compact or microcrystalline, give rise to a felspar porphyry; while crystals or rounded grains of quartz in a similar felsitic base produce a quartz-porphyry or quartz-fel-site. By Cornish miners these quartz-porphyries are termed elvans (elvanite of Jukes); but this name is also applied to fine-grained granites and to almost any rock which occurs as a dyke running through the killas or clay slate.





Few questions have been more warmly discussed than the origin of granite. When this rock is found forcing its way through older rocks, and appearing at the surface in large bosses from which veins are sent forth in all directions, there can be little doubt of its eruptive character. The small width of some of these granitic veins, or apophyses, suggests that the rock must have existed in a condition of perfect fusion or complete liquidity, and not simply as a viscous paste, before it could have been injected into such narrow fissures as those which are now occupied by granite. In many cases, the rocks which are penetrated by the granitic veins are altered in such a manner as to indicate a considerable elevation of temperature : a limestone in the neighbourhood of the veins may become saccharoidal, and shales may become indurated or even converted into horn-stone, while new minerals are often developed in the vicinity of the intruded veins. In these veins the granite is apt to change its mineralogical constitution, becoming either fine-grained or felsitic, or even reduced at the extre-mities of the vein to quartz. From the days of Hutton it has been generally admitted that most granite is of igneous origin. Since it appears to have been solidified at great depths beneath the surface, it has been distinguished as a plulonic rock, while those eruptive rocks which have risen to the surface, and have there consolidated, are termed volcanic rocks. The older geologists regarded granite as the primitive rock of the earth's crust, forming the floor of all stratified deriosits and the nucleus of mountain chains. Such a view, however, has been long exploded. It is known indeed that granite, so far from being in all cases an original rock, may be of almost any geological age. Some is undoubtedly as old as the Silurian period, while other granites are certainly as young as the Tertiary rocks, and perhaps of even more recent date. By many field-geologists granite has of late years been regarded as a metaniorphic rather than as a truly igneous rock. Meta-morphisin, however, is a term which has been so vaguely used that most of our eruptive rocks may, in a certain sense, be said to be metaniorphic. Still, in the case of granite, it has often been pointed out that a passage may be traced from this rock into gneiss, and that gneiss itself may be regarded as an altered sedimentary rock. Thus so experi-enced an observer as Professor Ramsay expresses his opinion that " granite is sometimes merely gneiss still further metamorphosed by heat in the presence of moisture" (Phys. Geol. of Gt. Brit., 5 ed., 1878, p. 42). For a number of instances in which granite is said to pass into gneissose rocks, and these in turn, by numerous gradations, into un-doubtedly stratified deposits, see Green's Geology, part i p. 307, and also GEOLOGY, vol. x., p. 309.

Chemists have also brought forward arguments against the igneous origin of granite. Thus it has been argued that the specific gravity of the quartz of granite is about 2-6, while that of silica after fusion is only 2'2. It must be remembered, however, that the quartz of granite has solidified under great pressure, as proved by Mr Sorby's observations, and it is probable that such pressure would increase the density of the silica. Moreover, it has been pointed out by the late D. Forbes (Geol. Mag., iv., No. 10, 1867, p. 443) that the siliceous tests of certain infusoria, which assuredly have not been fused, are as low as 2-2. Another argument which has been advanced against the igneous origin of granitic rocks is based on the fact that they contain minerals of a basic character which could not have existed in a state of fusion in the presence of free silica, without forming a combination with the latter. Again, some of the accessory minerals in granite would suffer change by an elevation of temperature, while many of them contain water which, it is assumed, would be expelled on fusion. Probably, however, these minerals are in most cases of secondary origin, and have been produced by the alteration of the granite. The mere presence of water is not incompatible with a pyrognostic origin; and Forbes has asserted (op. cit.) that specimens of lava, taken from a current on Etna while the lava was still flowing, contained crystals of stilbite, a mineral containing 16 per cent, of water. Mr Sorby has shown, too, that the quartz of vol-canic rocks contains microscopic cavities enclosing liquid. It appears, indeed, that in the fusion of all eruptive rocks water has played a very important part. Dr Haughton has sought to reconcile the opposing views as to the origin of granite by admitting what he calls a hydrometammphic origin for this rock. He believes that the rock, having been poured into veins and dykes when in a state of fusion, was subsequently altered by the action of water at tempera-tures which, though high, were insufficient for the fusion of the granite (On the Origin of Granite : an Address to the Geol. Soc. of Dublin, 1862).

Granitic rocks are extensively used for constructive and decorative purposes, though their industrial applications are necessarily restricted by the expense of working so hard a material. Although some granites are apt to decompose on exposure to atmospheric influences, the felspar passing into china-clay, other varieties are remarkable for their extreme durability, as attested by the monuments of ancient Egypt, on which the incised hieroglyphics still retain their eharpness. It appears that in England granite was not brought into extensive use much before the beginning of the present century (Creasy). It is now largely employed for massive structures, such as bridges and sea-walls, as also for kerbs and paving-sets. The best known granites are the grey variety from Aberdeen and the red granite of Peter-head, 30 miles north of Aberdeen. The granite of Dart-moor in Devonshire, and of the huge bosses which protrude through the clay slate of Cornwall, are largely worked as building and ornamental stones (for description of these granites see Sir H. T. de la Beche's Report cm the Geol. of Cornwall and Devon). In Ireland there is much fine granite, which is quarried to a limited extent,—the Castle-wellan granite having been used in the Albert Memorial in Hyde Park (Hull's Building and Ornam. Stones, 1872). The hornblendic varieties of granite are remarkably tough, and are largely employed for road-metal. For this purpose great quantities are quarried in the Channel Islands. Red granite occurs in abundance on the coasts of Maine, U.S., as well as in New Brunswick, and grey granite at Quincy and elsewhere in Massachusetts. Granite is fre-quently polished when used for monumental and decorative purposes. The polishing is effected by an iron tool, worked first with sand and water, then with emery, and lastly with putty-powder or oxide of tin ; when the emery and putty are used, a surface of flannel is interposed between the granite and the iron tool (" On Granite Working," by Geo. W. Muir, Journ. Soc. Arts, xiv., 1866, p. 470).
As an element of scenery granite generally forms rounded hills, scantily clad with vegetation; but it sometimes rises into sharp pinnacles, as in the aiguilles of the Alps. By denudation the rock may break up into cuboidal blocks, which often remain piled upon each other, forming the characteristic " tors " of Cornwall and Devon. Hills of granite are frequently surmounted by masses of weathered rock of spheroidal form, such as the Cornish rocking-stones or logan stones. The weathering of granite often produces boulder-shaped masses in sucli nunibers as to form, around the summit of the hill, a " sea of rocks " (Felsenmeer).

In addition to the references given in the body of this article, the following authorities may be cited :—Lehrbuch der Pétrographie, by F. Zirkel, 2 vols., Bonn, 1866; Elemente der Pétrographie, by Von Lasaulx, Bonn, 1875 ; Cotta's Lithology, translated by P. H. Lawrence, 2d ' ed., London, 1878; Tlte Study of Bocks, by F. Butley, London, 1879; and the numerous contributions by the Rev. Prof. Haughton in Proc. Irish Acad., and Quart. Journ. Geol. Soc. Lond. For chemical composition of Cleopatra's Needle, by G. W. Wigner, see the Analyst, 1878, p. 382. (F. W. R. *)




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