M. T. Rill — Cretaceous Formations of Mexico. 317 



thousand feet was apparently the medial part of the wide 

 oceanic strip that separated the two American continents in 

 early Cretaceous times and probably the region of greatest 

 deposition. This early Cretaceous subsidence and the accom- 

 panying accumulation of sediments was so great as to almost 

 obscure the pre- Cretaceous history of the Mexican peninsula 

 for to-day there are but few places where these limestones, 

 even though aided by the enormous disturbance, have been 

 eroded through to the pre-Cretaceous floor ; yet by and of 

 these there is enough evidence to show that the Mexican 

 peninsula had its outline in even earlier geologic time. 



The second great event in the history of the Mexican Cre- 

 taceous was the elevation of the sea-bottom and the return to 

 land conditions of the region at the close of the Comanche 

 sub-epoch, — an event most important in the interpretation of 

 the history of all the American Cordilleran region and one 

 which has been overlooked owing to the more distinct land 

 making after the close of the Upper Cretaceous. The evi- 

 dence of this mid-Cretaceous land is : (1) The unconformable 

 deposition of the Upper Cretaceous upon the Comanche Series 

 and consequent discordant overlaps of its western margin. 

 (2) The entirely different outline of the western littoral of the 

 Upper Cretaceous deposits of the Atlantic deposition in the 

 Cordilleran region as shown upon the map, showing that there 

 had been disturbances of the continental area in the Cordille- 

 ran region between the intervals of their deposition. (3) The 

 evidence of an extensive land area during Upper Cretaceous 

 time throughout the great basin region of Mexico and the 

 United States, from which was derived the vast accumulation 

 of Upper Cretaceous littoral plant-bearing sediments along the 

 eastern margin of this mid-Cretaceous land. The entire dis- 

 similarity and otherwise inexplicable difference between the 

 faunas of the Atlantic and the Pacific Upper Cretaceous sedi- 

 mentation south of the Canadian border indicate a continental 

 barrier between them in Cretaceous time. 



S. F. Emmons* has recognized in the Colorado division of 

 the Eocky Mountains several areas of what he calls " Jurassic 

 land " and cotemporary movements. These may have extended 

 into the early Cretaceous, for the earliest marine datum of 

 subsidence in that region by which he was able to fix the date 

 of this land area was the basement littoral of Dakota of the 

 Upper Cretaceous. 



Between the two epochs the ocean over the Texan region 

 no doubt receded nearly to its present outline as shown by the 

 area traversed by the Dakota littoral from eastern Texas to 



* Orographic movements in the Rocky Mountains, Bull. G-eol. Soc. Am ., vol. 

 i. pp. 245-286. 



