﻿I860.] 



FORBES— BOLIVIA AND PERU. 



53 



10. Silurian Formation. — The rocks which T have grouped together 

 as pertaining to the Silurian epoch show themselves continuously, or 

 very nearly so, over an extent from north-west to south-east of more 

 than 700 miles ; and the area occupied by them cannot be estimated 

 at less than 80,000 to 100,000 square miles. They form the moun- 

 tain-chain of the high Andes, rising to an absolute height of 25,000 

 feet above the sea, and, in the part of South America more parti- 

 cularly the subject of this memoir, continuous through Peru from 

 the north of Cusco over the snowy ranges of Carabaya and Apollo- 

 bamba, across the provinces of Munecas, Larecaja, La Paz, Yungas, 

 Sica-Sica, Inquisivi, Ayopaya, Cochabamba, Cliza, Misque, Chayanta, 

 Yamparez, Porco, Tomini, and Cinti, throwing off spurs along the 

 eastern side of the main chain, right through the province of Cau- 

 polican, down to the River Beni in Mojos, into Yuracores, Valle 

 grande, Santa Cruz and Chuquisaca, and to the east into the pro- 

 vinces of Oruro, Potosi, and Chichas*. 



Some of the greatest rivers of the world have their sources in this 

 mountain-chain. The Amazon, with its mighty affluents the Purus, 

 Madera, Beni, Mamore, Rio Grande, as well as the Pilcomayo and 

 other branches of the River Plata, are feci by the snows of this great 

 Silurian region. 



In this range also we meet with the loftiest mountains of South 

 America, second in height only to the Himalayas. Thus we have 

 Illampu (Sorata) 24,812 feet (25,200, Pentland), Illimani 24,155 

 (24,200, Pentland), Huayna Potosi 21,883, Coloolo 22,374, and 

 many others, rivalling these in height, but the elevation of whose 

 peaks has never as yet been ascertained. These peaks do not, as 

 M. D'Orbigny's published researches would lead us to suppose, con- 

 sist of mighty cones or bosses of granite, but are in reality composed 

 of Silurian strata, — fossiliferous, as I have proved in the case of 

 Illampu (the highest of them all) up to its very summit f . 



The Silurian series in these regions present a physical configura- 

 tion, as well as other features, so unmistakeably analogous to those of 

 their equivalents in Europe, that, notwithstanding the much grander 

 scale on which they are developed, the geologist cannot but imagine 

 himself breathing the air of Siluria, even before an examination of 

 the rocks themselves confirms this suspicion. 



The extensive development of clay-slate, shales, and grauwackes, 

 along with the metallic contents of these rocks, present mineral cha- 



* I have visited, of course, but a small portion of this vast territory, but have 

 availed myself of all procurable data and many specimens of the rocks of these 

 provinces, principally from mining adventurers and others who have explored 

 these districts, from whom I have received much information and assistance. 



f M. D'Orbigny presents us in section with the lofty Illimani as an immense 

 cone of granite. When there, I could not find a trace of this rock ; and Mr. 

 Pentland, who ascended the side of Illimani to a much greater elevation than 

 I did, assures me that he met with only clay-^slate, and found no trace of granite 

 or other eruptive rock. Mr. Horner has also directed my attention to the an- 

 nexed paragraph in Naumann's ; Lehrbuch der Geognosie,' 2nd edit. 1858, vol. i. 

 p. 97: — "Die beiden hochsten Gipfel der ostlichen Andeskette von Bolivia, 

 namlich der Sorata (19,974 F.)und der Illimani (19,834 F.), bestehen aus Grau- 

 wackenschiefer, und sind keine Vulcane." 



