54 Geology of South America. 



refers, indeed, to this deposit, the whole tertiary beds of Pata- 

 gonia, which are marine, but also contain some terrestrial or 

 fluviatile organic remains, transported probably by rivers. 

 He also includes under it the marine deposits of the province 

 of Entre-Rios ; and, in comparing these with the deposits of 

 Patagonia, he finds that the two groups are divisible into the 

 following series : — 1. A lower portion, composed of marine 

 sandstones, containing extinct species of mollusca. 2. A little 

 higher up, there are in both groups sandstones, in which bones 

 of mammiferse and pieces of fossil wood are met with. 3. To 

 this succeed, in the north, alternations of sandstone and clay 

 abounding in gypsum, and, in the south, blue sandstones. 

 4. The upper portion, in the north as well as the south, con- 

 sists of an alternation of limestones and sandstones containing 

 the Ostrea Fatagonica ; and, reposing on them, marine con- 

 glomerates, including in both groups, and thus at a distance of 

 upwards of 600 miles, three identical species of fossils which 

 prove their contemporaneity. There is thus every where an 

 analogy, not only in relative thickness and composition, but 

 also in organic remains ; and this similitude has determined 

 M. d'Orbigny to regard the whole as belonging to one and the 

 same epoch. 



M. d' Orbigny made many curious observations on the Pata- 

 gonian tertiary formation \ but of these we shall only cite a 

 few. At Ensenada de Ros, to the south of the Rio Negro in 

 Patagonia, one of the beds of sandstone presented a great num- 

 ber of bones, which, however, the hardness of the rock fre- 

 quently prevented him from obtaining. Among other remains 

 he discovered bones of the Megamys PatagonensiSy an animal 

 four times larger than any living species of the order Rodentia. 

 These consisted of a tibia with its rotula, whose relative posi- 

 tion with regard to each other was such, as to shew that they 

 must have been deposited while their ligaments still caused 

 them to adhere together. They were found under a mass of 

 jtnarine sandstones, containing shells and beds of oysters, more 

 than 650 feet thick. These oysters all belong to one species, 

 of which the beds occupy a very well-marked region in the 

 province of Entre-Rios, as well as along the whole coast of 

 Patagonia. It is evident that these shells lived together in 



