1902 Encyclopedia > Spain > Spain - Geology

(Part 3)


Spain - Geology

The geological structure of the Spanish Peninsula is Geology, comparatively simple. Upon a fundamental platform of ancient crystalline rocks, which had previously been upraised into detached ridges, a series of sedimentary formations was laid down, among which occur representatives of most of the geological systems from the older Palaeozoic rocks up to those of Quaternary date. Arranged in order of age, with their respective areas, these various groups of rock are shown in the subjoined table :—

== TABLE ==

Archaean rocks are exposed in the northern half of the Peninsula, particularly along the great Pyrenean axis, in Galicia, Estremadura, the Sierra Morena, the Sierra Nevada, and Serrania de Ronda. They consist of granites, gneisses, and mica-schists, with talc-schists, amphibolites, and crystalline limestones. The oldesl Palseozoic strata are referred, from their included fossils, to the Cambrian and Silurian divisions. They range through a vast region of Andalusia, Estremadura, Castile, Salamanca, Leon, and Asturias, and along the flanks of the Pyrenean and Cantabrian chain. They consist of slates, grey wackes, quartzites, and diabases. Grits, quartzites, and shales referable to the Devonian system occur in a few scattered areas, the largest and most fossiliferous of these occurring in the Asturias. The Carboniferous rocks of Spain are divisible into three groups, the lowest consisting of limestones with sandstones and shales, the middle of conglomerates and sandstones, and the upper of sandstones, conglomerates, shales, and coals. They lie in detached basins, and have not yet been well explored. One of these areas covers a considerable space in the Asturias, whence it stretches more or less continuously through the provinces of Leon, Palencia, and Santander, covering altogether an area of 6500 square kilometres. Another tract occurs at San Juan de las Abadesas in Catalonia, where it occupies about 200 square kilometres ; while a third, about 500 square kilometres in extent, runs from the province of Cordova into that of Badajoz. There are other smaller areas containing little or no coal, but showing by the included plant-remains that the strata undoubtedly belong to the Carboniferous system.

The Triassic system is well developed in the north of the Peninsula along the Cantabrian chain and eastwards to the Mediterranean. It is composed of red and variegated sandstones, dolomites, and marls, traversed in some places by ophitic rocks, and containing deposits of gypsum, aragonite, and rock-salt. These strata are overlain by members of the Jurassic series, which are especially conspicuous in the eastern part of the Peninsula between Castile and Aragon, along the Mediterranean border, in Andalusia, and likewise along the flanks of the Pyrenees. The Lias is best represented. The Cretaceous system is distributed in four great districts: the largest of these extends through the kingdoms of Murcia and Valencia; a second stretches between the two Castiles; a third is found in the Basque Provinces and the Asturias; and a fourth spreads out along the southern slopes of the Pyrenees from Navarre to the Mediterranean. The lower members of the Cretaceous series include an important freshwater formation (sandstones and clays), which extends from the Cantabrian coast through the provinces of Santander, Burgos, Soria, and Logrofio, and is supposed to represent the English Wealden series. The higher members comprise massive hippurite limestones, and in the Pyrenean district representatives of the upper subdivisions of the system, including the Danian.

Deposits of Tertiary age cover rather more than a third of Spain. They are divisible into two great series, according to their mode of origin in the sea or in fresh water. The marine Tertiary accumulations commence with those that are referable to the Eocene series, consisting of nummulitic limestones, marls, and siliceous sandstones. These strata are developed in the basin of the Ebro, and in a belt which extends from Valencia through Murcia and Andalusia to Cadiz. Marine Miocene deposits occupy some small tracts, especially on the coast of Valencia. But most of the sandy Tertiary rocks of that district are Pliocene. The Tertiary masses of Andalusia have coarse conglomerates (Middle Miocene) at their base, followed by thick beds of Bryozoan molasse and younger (Pliocene) beds. These strata are specially noteworthy for containing an important metalliferous deposit, that of the native silver of Herrerias, which is found in a Pliocene bed in the form of flakes, needles, and crystals. But the most extensive and interesting Tertiary accumulations are those of the great lakes which in Oligocene and Miocene time spread over po large an expanse of the tableland. These sheets of fresh water covered the centre of the country, including the basins of the Ebro, Jucar, Guadalaviar, Guadalquivir, and Tagus. They have left behind them thick deposits of clays, marls, gypsum, and limestone, in which numerous remains of the land-animals of the time have been preserved.

Quaternary deposits spread over about a tenth of the area of the country. The largest tract of them is to be seen to the south of the Cantabrian chain ; but another, of hardly inferior extent, flanks the Sierra de Guadarrama, and spreads out over the great plain from Madrid to Caceres. Some of these alluvial accumulations indicate a former greater extension of the snowfields that are now so restricted in the Spanish sierras. Remains of the reindeer are found in caves in the Pyrenees.

Eruptive rocks of many different ages occur in different parts of Spain. The most important tract covered by them is that which stretches from Cape Ortegal to Coria in Estremadura and spreads over a large area of Portugal. They likewise appear in Castile, forming the sierras of Gredos and Guadarrama; farther south they rise in the mountains of Toledo, in the Sierra Morena, and across the provinces of Cordova, Seville, Huelva, and Badajoz as far as Evora in Portugal. Among the minor areas occupied by them may be especially mentioned those which occur in the Triassic districts. Of rocks included in the eruptive series the most abundant is granite. There occur also quartz-porphyry (Sierra Morena, Pyrenees, &c), diorite, porphyrite, diabase (well developed in the north of Andalusia, where it plays a great part in the structure of the Sierra Morena), ophite (Pyrenees, Cadiz), serpentine (forming an enormous mass in the Serrania de Ronda), trachyte, liparite, andesite, basalt. The last four rocks occur as a volcanic series distributed in three chief districts—that of Cape Gata, including the south-east of Andalusia and the south of Murcia, that of Catalonia, and that of La Mancha.

Read the rest of this article:
Spain - Table of Contents

About this EncyclopediaTop ContributorsAll ContributorsToday in History
Terms of UsePrivacyContact Us

© 2005-23 1902 Encyclopedia. All Rights Reserved.

This website is the free online Encyclopedia Britannica (9th Edition and 10th Edition) with added expert translations and commentaries