124 Geology of South America. 



basin of the Pampas. There is thus attributed to this deposit 

 an origin analogous to that which has often been attributed in 

 Europe to a portion of the plastic clay formation. The absence 

 of fossils in the Guaranian deposit, its invariable ferruginous 

 nature, and its imperfect stratification, would seem favourable 

 to this supposition. 



A new period of repose having then succeeded to the dis- 

 turbances, the tertiary seas extended to the east and to the 

 west of the Chilian system. The marine sedimentary deposits 

 of the Patagonian formation began to extend over the Guaran- 

 ian formation. Terrestrial streams transported from the 

 neighbouring continents bones of mammifera, fragments of 

 wood, and fluviatile shells ; some of them coming, no doubt, 

 from the ridge of the Chilian system, would convey from the 

 SE. bones, still provided with their ligaments, into the Pata- 

 gonian sea, while others arrived from the great northern con- 

 tinent, that is to say, from Brazil, which had already emerged 

 in a great measure from the ocean. The continent of South 

 America already possessed, so to speak, in the state of outline, 

 the configuration which it was to preserve ; it already present- 

 ed a chain rising above the ocean, indicating the course of the 

 Cordillera from N. to S., and thus separating the Atlantic Ocean 

 from the Pacific Ocean by a narrow tract of land, similar to 

 the isthmus of Panama of the present day. We can thus con- 

 ceive how the tertiary formation of the two sides may have 

 been contemporaneous, although they do not contain species 

 of fossil shells common to both ; and, notwithstanding the 

 reservations which we have made above, it must be confessed 

 that the hypothesis proposed by M. D'Orbigny, explains so 

 happily the complete difference of the Faunas of these two 

 formations, of at least a nearly similar age, that it is difiicult 

 not to regard it as possessing a very considerable degree of 

 probability. 



But the seas which then encroached so largely on the form 

 ultimately assumed by South America, receded and removed 

 from the base of the Cordillera, leaving the continent to in- 

 crease in size, towards the east, by the amount of all the space 

 occupied by the Patagonian tertiary formation, and towards 



