Cretaceous Period. 51 



same districts, have recalled to M. d'Orbigny the aspect of 

 the rocks of Bolivia, which he compares with them. The 

 American formations, whose nature and position thus lead us 

 to refer them, at least provisionally, to the Trias, but whose 

 paleontological characters are still almost unknown, seem re- 

 duced to occupy, at the present day, and that in the form of 

 large detached portions, the two slopes of the eastern Cordil- 

 lera of the Bolivian system, and they there attain, at their 

 highest elevation, a height of 4000 yards above the level of the 

 sea. They are probably the remains of a great whole which 

 covered the surface before the occurrence of the geological 

 catastrophes that impressed the present forms on its exter- 

 nal physiognomy. 



Absence of the Jurassic System. — One of the most remark- 

 able circumstances in American Geology is the absence of the 

 Jurassic formations, a fact announced a long while ago by Von 

 Buch. M. d'Orbigny did not find a single fossil which seems 

 to belong to that period. The only exception to the general 

 rule hitherto brought forward is, that M. d''Orbigny saw some 

 Jurassic TerebratuIjE among the fossils collected by M. Do- 

 meyko from a limestone in Chili. 



Cretaceous Period, — The deposits of the cretaceous period 

 seem, on the contrary, to have been very much developed on 

 the American continent, as is proved by the collections of fos- 

 sils made by Von Humboldt, Boussingault, Degenhardt, and 

 by the geologists of Dumont d'Urville's last voyage. Doctors 

 Hombron and Le Guillou. They occur from Columbia to 

 Tierra del Fuego, or over the whole length of South America, 

 whilst nevertheless they are interrupted in the middle. At 

 that epoch there lived in America as in Europe, particular 

 forms of Ammonite and Ancyloceras, &c. ; and independently 

 of the general resemblance of the forms, there were in Co- 

 lumbia and in the Parisian basin enough of identical species 

 to induce us to suppose, that there was a direct communication 

 between the European and the Columbian portions of the 

 Cretaceous sea. It is well known that this sea formed in 

 France two great distinct basins, the Parisian and the Medi- 

 terranean. It appears that this same sea covered with its 

 waters not only a considerable portion of Cohunbia, but gene- 

 rally a large part of the regions situated on the north, the 



