. Elements of geology. Geology. 286 LYELL'S ELEMENTS OF GEOLOGY. Plutonic Rocks of the Chalk, Oolite, and Lias. tary rocks of different ages, the strata in both resting on plutonic rocks, by which they have been altered. In the western or old- est range, called the Peuquenes, are black calcareous clay-slates, rising to the height of nearly 14,000 feet above the sea, in which are shells of the genera Gryphsea, Turritella, Terebratula, and Ammonite. These rocks are supposed to be of the age of the central parts of the secondary series of Europe. They are penetrated and altered by dikes and mount
. Elements of geology. Geology. 286 LYELL'S ELEMENTS OF GEOLOGY. Plutonic Rocks of the Chalk, Oolite, and Lias. tary rocks of different ages, the strata in both resting on plutonic rocks, by which they have been altered. In the western or old- est range, called the Peuquenes, are black calcareous clay-slates, rising to the height of nearly 14,000 feet above the sea, in which are shells of the genera Gryphsea, Turritella, Terebratula, and Ammonite. These rocks are supposed to be of the age of the central parts of the secondary series of Europe. They are penetrated and altered by dikes and mountain masses of a plutonic rock, which has the texture of ordinary granite, but rarely contains quartz, being a compound of albite and horn- blende. The second or eastern chain consists chiefly of sandstones and conglomerates, of vast thickness, the materials of which are derived from the ruins of the western chain. The pebbles of the conglomerates are, for the most part, rounded fragments of the fossiliferous slates before mentioned. The resemblance of the whole series to certain tertiary deposits on the shores of the Pacific, not only in mineral character, but in the imbedded hg- nite and silicified wood, leads to the conjecture that they also are tertiary. Yet these strata are not only associated with trap rocks and volcanic tuffs, but are also altered by a granite newer than that of the western chain, and consisting of quartz, felspar, and talc. They are traversed, moreover, by dikes of the same granite, and by numerous veins of iron, copper, arsenic, silver, and gold ; all of which can be traced to the underlymg granite.* We have, therefore, strong ground to presume that the plutonic rock, here exposed on a large scale in the Chilian Andes, is of later date than certain tertiary formations. Cretaceous period.—It was stated (p. 145.) that chalk as well as lias have been altered by granite in the eastern Pyre- nees. Whether such granite be cretaceous or tertiary cannot easil
Size: 2277px × 1098px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorlyellcharlessir17, bookcentury1800, booksubjectgeology
