212 



bones of the Megatherium and ol'five or six other species of quadru- 

 peds, among which he has detected the bones of a species of Agouti. 

 He also met with several examples of the polygonal plates of the Me- 

 gatherium^ which at first induced him to regard the animal as a gigan- 

 tic Armadillo. A very large collection of these fossils has been sent 

 to England, and are in the custody of Professor Henslow till Mr. Dar- 

 win's return. 



Professor Sedgwick concluded by reading extracts from two letters 

 describing a section transverse to the Andes, extending from Valpa- 

 raiso to Mendoza. The Cordillera is here composed of two separate 

 and parallel chains. The western chain is composed of sedimentary 

 rocks, distinctly stratified, and resting on granite. The sedimentary 

 rocks (composed of red sandstone, conglomerate, gypsum, &c.) are 

 violently contorted, and dislocated along parallel north and south 

 lines, and as they approach the granite, become so crystalline that they 

 cannot be distinguished from the porphyritic dykes by which they are 

 traversed. 



Following the line of section, Mr. Darwin found, at the Pass of 

 Puquenas, elevated 12,000 feet above the sea, that the red sandstone 

 was replaced by a black rock, like clay slate and pale limestone, con- 

 taining numerous impressions of shells ; a Gryphsea? is the most abun- 

 dant; but he also found Ostrea, Turritella, Ammonites, and a small 

 bivalve (Terebratula?). 



At the Portillo Pass is a conglomerate resting on micaceous sand- 

 stone, and traversed by great veins of granite. But at the Uspellata 

 Pass (in the eastern chain), he found highly crystalline and felspa- 

 thic rocks, regularly bedded, and resting on granite, the peaks of 

 which reach the elevation of 14,000 feet. A wider examination of 

 the overlying groups convinced him, not only that they were more re- 

 cent than the western chain (being partly made up of its debris), but 

 that they were of the same age with certain tertiary formations above 

 noticed. For example, he discovered along the line of section, in 

 the eastern chain, beds of sandstone, with silicified trunks of dico- 

 tyledonous trees, and beds of carbonaceous shale, resting on an an- 

 cient stream of lava, and surmounted byblack augiticlava, 2000 feet 

 thick ; over all these were five grand alternations of black volcanic 

 rocks and sedimentary deposits, amounting to several thousand feet 

 in thickness. This series, in its structure and fossils, is considered as 

 identical with certain tertiary deposits of Patagonia, Chiloe and Con- 

 ception ; for it loses its mineral character only where it approaches 

 the granite; in which case it is shattered, contorted, and traversed by 

 great veins rising out of the central mass; and its several beds, as 

 well as the fossils they contain, become entirely crystalline. Mr. Dar- 

 win further states, that this singular overlying group contains very nu- 

 merous veins of copper, silver, arsenic, and gold, which maybe traced 

 down to the granite ; and as a general conclusion, he expresses his 

 conviction that the granite (now rising into central peaks 14,000 feet 

 in elevation), must have been in a fluid state since the tertiary group, 

 above described, was deposited. 



