HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 259 
tions. We can now clearly separate the Cretaceous formations of the 
United States into two distinot but synchronous provinces, — those of 
Atlantic and Pacific sedimentation respectively. 
In the State of San Luis Potosi the basal (Neocomian) beds of the 
Pacific Cretaceous fauna are found lying beneath those of the Comanche 
(Neocomian) of the Atlantic sedimentation of the Texas region. Above 
this point there is no trace of community of fossil forms of the Atlantic 
and Pacific regions. It has been argued that near the close of the 
Upper Cretaceous the great Gulf of Mexico, which at that time spread 
over much of the present Great Plains region and the Rocky Mountain 
front, may have had a northern connection in British Columbia through 
a shallow passage with the Pacific, yet was still separated to the south- 
ward; but there are reasons for doubting even this connection. 
This dissimilarity between the Cretaceous faunas of the two coasts 
apparently continued throughout the entire length of the period. “No 
geographic continuity of their strata has yet been observed, and no 
satisfactory recognition of their faunal relationship has been made.” ! 
Furthermore, the Cretaceous formations of the Pacific coast of the 
North and South American regions are underlain by Jurassic formations, 
and are stratigraphically inseparable from them, and even to-day pale- 
ontologists are debating the line of limitation between them, while on 
the Atlantic side, in the United States at least, the lowest Cretaceous is 
everywhere, so far as known, unconformably deposited on a pre-existing 
land area, composed of geologic formations of various ages. 
In South America, although the faunas and formations have not been 
so extensively studied, apparently the same great dissimilarity existed 
in Cretaceous time between the life of the Pacific and Atlantic waters. 
Of the three hundred or more species from the Pacific sediments, it is 
doubtful if any are identical with the forms found in Brazil or the West 
Indies. 
The Cretaceous faunas of the Pacific coast of North America contain 
a rich fauna of abundant species having an individuality of its own, 
closely related to the Trans-Pacific faunas of Russia and Western Europe, 
but absolutely distinct from the synchronous beds deposited on the other 
side of a great continental barrier in the waters of the Atlantic. 
On the Atlantic side of the Andes in Colombia and Ecuador and parts 
of Brazil, the little that is known of the Cretaceous fauna is thoroughly 
suggestive that it is largely eastern in its facies, presenting some resem- 
blances to the Atlantic Cretaceous of the United States, which, when the 
1 C. A. White, “ Correlation Papers, Cretaceous,” pp. 197, 198. 
