mesozoic time — triassic and jurassic. 739 



1. Teiassic and Jurassic Periods. 

 AMERICAN. 



Tlie orographic events in North America closing Paleozoic time changed 

 greatly the areas of future rock-making. The map on page 735 shows that no 

 marine deposits were possible on the Atlantic border except far outside of 

 the present coast-line. Moreover, as announced on page 734, all of the east- 

 ern half of the continent had become dry land, leaving only the western 

 half covered with the Interior Continental Sea, and therefore as the great 

 arena of progress. The mostly emerged condition of the Atlantic border, 

 indicated on the map, continued through the Triassic and Jurassic periods 

 and after the Cretaceous period had opened ; for the beds of the Upper Cre- 

 taceous are the earliest Mesozoic marine deposits on the border. Before 

 this, however, in the Triassic period, there were large estuary and fresh-water 

 deposits in progress, and these constitute the Triassic formation of the 

 Atlantic border. 



In this condition of the continent, the regions of Mesozoic rock-making 

 were the following : (1) the Atlantic border area ; (2) the Gulf border area; 

 (3) the area of the Western Continental Interior; (4) the Pacific border; 

 (5) the Arctic area, Arctic rock-making continuing to be independent of that 

 over the North American continent in changes of level and in the 

 formations produced. 



The Pacific border comprises four belts, ranging from northwest to south- 

 east, which were more or less independent in their geological history : — 



I. The Rocky Mountain belt, which includes in British America the 

 Archaean protaxis and the adjoining upturned or mountain region situated 

 mainly to the east of the protaxis, comprises over the United States the 

 wide summit region between the Great Basin and the eastern foothills of 

 the Pront or Colorado Range, 



II. The Plateau belt, or that of the Great Basin, with its continuation 

 northward in British Columbia over the interior plateau west of the Gold 

 Eange or Protaxis ; and southward into Mexico, along the corresponding 

 plateau region. 



III. The Sierra belt, or that of the Sierra chain, including the Sierra 

 Nevada, the Cascade Eange, and the high ridges in the same line through 

 British Columbia. 



IV. The Coast belt, or that of the Coast Range of California and Oregon, 

 and the Island Range of British Columbia. 



The interval between the Sierra and Coast ranges, also, is in some respects 

 entitled to be considered a separate belt; but it is narrow, and its history 

 is mostly involved in that of these ranges. 



Only in western North America have the Triassic and Jurassic formations 

 been separately distinguished, and there at but few outcrops. Deposits of the 



