NEGATIVE CONTINENTAL' ELEMENTS 449 



The foregoing arrangement of these seas will probably impress the 

 reader in two ways : First, the great number of North American seas, and, 

 second, their rather free intercommunication. The multiplicity of seas, 

 which in the main were of Paleozoic time, unmistakably indicates shallow 

 bodies of water variously separated by more or less ineffective land bar- 

 riers. A survey of the paleogeographic maps presented in this paper will 

 make this fact abundantly evident, and it may be likewise observed that 

 not only did the marine waters flow in on the land from the four sides of 

 the North American continent, but that the seas were localized among 

 lands that suggest an archipelago of large islands. Further study will show 

 that the Paleozoic continental seas began in a small way, pulsated back and 

 forth over the continent, and, if a few irregularities are disregarded, in- 

 creased in area until they almost completely submerged North America in 

 Middle Ordovicic time. This great inundation was dominated by the Pa- 

 cific. The oscillatory nature of the seas continued, yet during the Siluric 

 the Arctic waters were the dominating force. With each recurring climax 

 of submergence, however, it is seen that the pulsations became smaller and 

 smaller until the close of the Paleozoic, when North America was again as 

 large as it had been at the beginning of this era. For a long period the 

 entire continent then remained positive except along the border region of 

 the Pacific, which ocean during the Triassic overlapped great areas and in 

 the late Jurassic developed the Logan sea. During this period, however, 

 contraction and subsidence of this immense ocean had gone on, and thrust- 

 ing now took effect, giving birth to the Sierra Nevadas. About this time 

 subsidence also took place over much of eastern Mexico, being probably 

 caused by the thrusting of the Pacific ocean indicated in the appearance 

 of the Sierra Nevadas. This thrusting was continued for a period equal 

 in length to the Comanchic, and resulted in the greater extension of the 

 Gulf of Mexico not only over the larger part of Mexico, but the syncline 

 stretched into the United States as far north as Kansas. A marked but 

 short withdrawal of this sea then took place, when the same syncline was 

 further extended, giving rise to the Coloradoan sea connecting the Gulf of 

 Mexico with the Arctic ocean. That this trough continued to subside is 

 shown by the fact that in Montana it contains about 12,000 feet of marine 

 deposits of Colorado and Montana age, and these series are said to be 

 followed by a similar thickness of Laramie and Livingston beds. In the 

 development of this trough must be assumed the gradual rise of the 

 Pocky mountains in the West, a considerable portion of whose elevation 

 has gone toward filling the syncline. 



Along the northern Atlantic thrusting culminated with the early Per- 

 mic revolution, since which time this ocean has gradually eaten its way 



