CRETACIC PERIOD 587 



In the Black Hills and in Wyoming are other continental deposits known 

 .■as the Lakota, Cloverly, and Fuson. In the Black Hills area about 1,000 

 large cycad trunks have been unearthed; these are larger than those from 

 the Patuxent of Maryland, but nearer them in development than the 

 smaller ones from the Morrison of the Freeze Out hills of Wyoming. 

 Knowlton informs the writer that these plant-bearing horizons practically 

 have one flora, which corresponds best with the Wealden of Europe. It is 

 closely linked with the Jurassic floras, as both have the same general 

 aspect and a number of species are common to the two floras. 



Cretacic Period 

 See plates 94 and 95 



The widely emergent condition of the North American continent dur- 

 ing the greater part of the Mesozoic was changed in the Cretacic. In the 

 Comanchic most of Mexico was beneath the sea, and much of its eastern 

 border so remained in the Cretacic, but here the invasion was far less 

 than before. The decided submergence of Cretacic time began with the 

 Dakota in the Gulf of Mexico area and spread north to the Arctic ocean 

 •east of the Eocky mountains. This formed the great Coloradoan sea, 

 a syncline which with continental deposits first made its appearance cer- 

 tainly as early as late Triassic time and was apparently due to the thrust- 

 ing of the Pacific border region during the early Mesozoic. The faunas 

 of this province are linked with those of the Gulf area and the Atlantic 

 border, but the last two regions have far more in common than either has 

 with the Coloradoan sea. The waters of the Gulf border and the Colo- 

 radoan sea came into existence far earlier than the Atlantic overlap. A 

 widely divergent faunal province occurs along the Pacific border from 

 Lower California to Alaska. There were therefore two very distinct faunal 

 provinces, and the one of the Atlantic is readily divisible into three sub- 

 provinces — the Coloradoan sea, the Gulf of Mexico region, and the Atlan- 

 tic border, the latter being most typical in New Jersey and Maryland. 



Throughout the Cretacic the Laramide range seems to have been in 

 slight upward movement, with decided elevation toward the close of this 

 period. The pressure coming from the Pacific folded the Cretacic de- 

 posits, which in places are 20,000 feet thick. As a result of this move- 

 ment, Forth America was again greatly enlarged, and for a short time 

 was connected with South America, thus permitting some intermigration 

 of the land animals of the two continents. 248 



248 For a digest of the literature up to 1890, see White, Bull. no. 82, U. S. Geological 

 Survey, 1891. 



