R. T.Hill — Cretaceous Formations of Mexico. 323 



beds at Nuevo Laredo, are found in the folded mountain struc- 

 ture, as are those of immediately preceding beds, and it is 

 probable that the age of the last folding of the Cordil- 

 leran eastern front can be accurately placed after the close of the 

 Laramie, during the Eocene in Mexico, as it is in the Rocky 

 Mountains. The movement no doubt began during the middle 

 of the Upper Cretaceous, when an elevation of the shore line 

 is indicated by the shallowing of the sediments, and was accel- 

 erated until the close of the Laramie, as is indicated in the 

 enormous accumulation of littoral latest Cretaceous and Lara- 

 mie sediments along the whole eastern half of the Rocky 

 Mountain and eastern Sierra Madre region, finally culminating 

 in the last great folding epoch immediately succeeding the 

 latter time, thus completing the cycle of subsidence and eleva- 

 tion that took place between the Comanche, or mid Cretaceous 

 and the Miocene. The waters of the arm of the Atlantic, or 

 attenuated Gulf of Mexico receded from the present areas of 

 the Great Plains for the last time, and the continental outline 

 of to-day was practically defined. 



How different is the land record of the Upper Cretaceous 

 epoch from that of the Lower Cretaceous. It is true that dur- 

 ing both periods the eastern margin of the continent again 

 subsided and the waters of the ocean invaded large land areas ; 

 but the shore line of this Upper Cretaceous-Laramie was 

 entirely different in the unstable Cordilleran region from that 

 of the Comanche epoch. During the latter epoch it was the 

 southern margin of the continent, or North American island, 

 that subsided, but in Upper Cretaceous it was the Central or 

 Great Plains region that went down, a great arm of the sea 

 having extended inland between the Cordilleran and Appa- 

 lachian lands, nearly severing our continent into two great 

 islands, as has been shown by Prof. Dana, and culminated in 

 the great littoral sedimentation of the Montana-Laramie epoch 

 along the east front of the narrow land strip of the nucleal 

 Cordilleran region. Nowhere in Mexico or the United States 

 were the waters of the two oceans connected, and while the 

 whole epoch was terminated in Eocene times by the last grand 

 peripheral folding of the Cordilleras as seen in the Rocky 

 Mountain front ranges from Montana to Mexico. 



The accompanying table shows the sequence of the Creta- 

 ceous formations of Mexico, and their relations to those of the 

 United States. 



Am. Jour. Sci. — Third Series, Vol. XLV, No. 268. — April, 1893. 

 23 



