318 R. T. Hill — Cretaceous Formations of Mexico. 



Wyoming, preceding the Upper Cretaceous subsidence which 

 covered the Great Plains and Rocky Mountain region, while 

 the heart of the Great Basin and the central part of Mexico 

 remained above sea level — apparently a continental divide trav- 

 ersing the heart of the entire central basin region of the Cor- 

 dilleras from Tehuantepec to the British boundary — and possi- 

 bly causing the great difference between the marine molluscan 

 faunas of the Pacific region and the Atlantic during Upper 

 Cretaceous times. This seldom appreciated and little under- 

 stood western continental strip represents to the writer a most 

 important stage in geological history, although but little topo- 

 graphic trace of it remains, for it was this land that furnished 

 the vast sediments accumulated along its eastern front in Cre- 

 taceous and Laramie time which were folded up in the last 

 grand epoch of Rocky Mountain making. 



The data on above map are adapted, after slight modification, from Senor An- 

 tonio Castillo's excellent Geological Map of Mexico. Mexico, 1889. 



The Upper Cretaceous Formation in Mexico. 



All writers on the Cretaceous formation in southern and 

 central Mexico have assumed that the mountain or " Hippur- 

 ites " limestone was the top of the Cretaceous series of the 

 country and have seemed unaware of the existence above it in 

 northern Mexico of the true or Upper Cretaceous formation, 



