Trachyte is almost as abundant as porphyry; many of the loftiest parts, and all the great dome-shaped mountains, are formed of it. Masses of this rock, from 14,000 to 18,000 feet thick, are seen on Chimborazo and Pichincha. Prodigious quantities of volcanic products, lava, tufa, and obsidian, occur on the western face of the Andes, where volcanos are active. On the eastern side there are none. This is especially the case in that part of the chain lying between the equator and Chile. The Bolivian Cordilleras, which encircle the valley of Desaguadero, furnish a striking example. The Cordillera of the coast is composed of crystalline and stratified rocks at its base, and of trachytes, obsidian, and trachytic conglomerates at greater elevations, while the eastern Cordillera consists of stratified rocks of the Silurian system, with granites, quartziferous porphyries, and syenites injected, and of secondary rocks of the triassic period, and marls, containing gypsum, oolitic limestone, and rock-salt of the most beautiful colours. Towards Chile, and throughout the Chilian range, the case is different, because active volcanos are there in the centre of the chain. Sea-shells of different geological periods are found at various elevations, which shows that many upheavings and subsidences have taken place in the chain of the Andes.[56] The whole range, after twice subsiding some thousand feet, was brought up by a slow movement in mass during the Eiocene period, after which it sank down once more several hundred feet, to be again uplifted to its present level by a slow and often interrupted motion. These vicissitudes are very perceptible, especially at its southern extremity. Stems of large trees, which Mr. Darwin found in a fossil state in the Uspallata range, on the eastern declivity of the Chilian Andes, now 700 miles distant from the Atlantic, exhibit a remarkable example of such vicissitudes. These trees, with the volcanic soil on which they had grown, had sunk from the beach to the bottom of a deep ocean, from which, after five alternations of sedimentary deposits and deluges of submarine lava of prodigious thickness, the whole mass was raised up, and now forms the Uspallata chain. Subsequently, by the wearing of streams, the embedded trunks have been brought into view in a silicified state, projecting from the soil in which they grew—now solid rock. “Vast and scarcely comprehensible as such changes must ever appear, yet they have all occurred within a period recent when compared with the history of the Cordillera; and the Cordillera itself is absolutely modern compared with many of the fossiliferous strata of Europe and America.”[57]