158 Geological Society. 
traverse above mentioned), Mr. Darwin crossed from Rio Negro to 
Buenos Ayres by Sierra de la Ventana, a chain almost unknown to 
geographers. He found two immense collections of large bones (of 
Mastodons) near Santa Fé, but in a condition not to admit of their 
being removed. He also found bones of a species of Mastodon at 
Fort St. Julian, S. lat. 50°, and more than 600 miles from the former 
localities. In one instance the bones appear to have been associated 
with marine shells. In the gravel of Patagonia he also found many 
bones of the Megatherium and of 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 tiese 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 Gryphza? 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 by black augitic lava, 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 
