Tertiary Formations. 56 



numbers, and have not suffered any derangement, for they are 

 every where met with in their natural position, and with their 

 two valves united. Judging from analogy, we may believe, 

 M. d'Orbigny remarks, that the basin in which they lived had 

 but little depth, and that the water did not rise more than 

 about thirty feet above these beds of oysters. The oysters, 

 like all the other shells met with in the tertiary beds of the 

 Pampas and of Patagonia, appear to M. d'Orbigny to be differ- 

 ent from those occurring at the present day in the same re- 

 gions. He thinks, indeed, that not one of these fossil species 

 is now to be met with alive. The bones of mammifera belong 

 likewise to extinct species, and even to extinct genera. 



The coasts of Chili are fringed, like those of Patagonia, by 

 a tertiary deposit that M. d'Orbigny did not himself exa- 

 mine, but which the notes and the collections placed at his 

 disposal by several travellers have enabled him to describe, 

 and of which he has determined and figured the fossil shells. 

 The fossil species of the tertiary beds of Chili (those from the 

 perfectly modern deposits excepted) no longer occur in a 

 living state on the same coasts. In this respect the ter- 

 tiary formation of Chili corresponds with the Patagonian ter- 

 tiary formation ; but it is a very curious fact, that, notwith- 

 standing this similitude, which would seem to refer them to 

 nearly the same geological period, these two formations, al- 

 though situated in the same latitude, do not contain fossils 

 common to both. Not only is there not a single identical 

 species, but even the series of genera is altogether distinct, — 

 a fact which appears to indicate, that, notwithstanding their 

 geographical approximation, these two formations have been 

 deposited in different seas. 



After having compared, in a paleontological point of view, 

 the tertiary formations of the two sides of South America, 

 M. d'Orbigny compares them in the same manner with those 

 of Europe, in order to endeavour to assign them an age in 

 the long series of tertiary periods. The result of this inves- 

 tigation is to establish that the following conditions apply 

 equally to the tertiary formations of the Parisian basin, and to 

 the tertiary formations of the two slopes of the Cordilleras. 

 1. None of the fossil species are met with in a living state on 



