M. Alcide d'Orbigny on Living and Fossil Mulluncs. t)3 



occasioned by the depths of the ocean, all of which have pre- 

 sented obstacles to the extension of river and pelagian faunas. 

 " If faunas have the same points of separation on differ- 

 ent continents, and if they are arrested by limits strongly 

 defined in their palaeontological composition, we must natur- 

 ally infer that the divisions of the formations do not depend 

 on partial causes, but that they arise from general causes 

 whose influence is felt over the whole earth. 



" From my researches in America, where geological facts 

 are observed on a large scale, I am led to believe that the 

 partial or total annihilation of faunas peculiar to each forma- 

 tion, always arises from the importance of dislocations pro- 

 duced on the surface of our planet by the contraction of the 

 matters owing to the cooling of the central parts,* and the 

 perturbations which have produced these same dislocations. A 

 system, or rather a chain of mountains, 50 degrees in length, 

 such, for example, as that of the Andes, of whose relief only 

 we can judge, without being able to calculate the correspond- 

 ing extent of its sinking in the bosom of the ocean, must 

 have caused such a movement in the waters, in consequence 

 of the displacement of matters, that the efi'ect would be uni- 

 versal both on continents and in seas. By this deluge land- 

 animals have been swept away from the former ; the trans- 

 portation of earthy particles has desolated the second, not 

 only suffocating the animals living at large in the ocean by 

 filling their branchiae, but also the more fixed animals of the 

 coast, by covering them up under a deposit. We may like- 

 wise suppose that a great disturbing cause has resulted from 

 the difft-rence of the levels formed along the whole shore of 

 oceans by this terrestrial movement. We may thus explain, 

 at the same time, the separation of beings by formation, and 

 their extinction at each of the great geological epochs. 



" The results of these dislocations being general over the 

 globe, and having manifested themselves at immense dis- 

 tances, we ought to seek in them the systems of elevation or 

 effect de bascule, ancient and modern, the cause of the annihi- 

 lation of the numerous faunas which have succeeded each 



* This is the opinion of M. Elii; de Bpauniont. 



