490 THE TRIASSIC PERIOD 



general, several degrees farther east than now, well up into British 

 Columbia. Between the latitudes of 55 and 60, the sea is believed 

 to have crossed the present Cordilleran belt. 



The published measurements assign the system the great thick- 

 ness of 17,000 feet (maximum) in the West Humboldt range of 

 Nevada, where it rests on pre-Cambrian terranes. To have sup- 

 plied, such a volume of sediment, the land to the east must have 

 been high, or repeatedly renewed, unless the great thickness is due 

 to oblique deposition. 



Climatic Conditions 



The wide distribution of gypsum and salt in the system, in more 

 than one continent, is evidence of wide-spread aridity. The prev- 

 alent redness of the system, in other continents as well as our own, 

 is also commonly regarded as an indication of aridity. Perhaps the 

 peculiarities of the Newark conglomerate may find their explanation 

 in such a climate, which favors great changes of temperature, and 

 so the disrupting of rock, if it is not covered by soil. Under such 

 circumstances, much coarse debris originates, largely of rock which 

 is undecomposed. Violent storms (cloud-bursts), which character- 

 ize some arid climates, might account for the transportation of 

 coarse debris from its place of origin to its site of deposition. For 

 the formation of abundant debris in this way, steep slopes are 

 needful, for gentle slopes and flats soon get a covering of mantle 

 rock which prevents the disruption of the rock beneath. If this was 

 the origin of the coarse materials of the conglomerate, their rounding 

 and wear would have to be attributed to the waves or currents of 

 the water in which deposition took place. 



Close of the Trias 



Considerable geographic changes marked the close of the Trias- 

 sic period in eastern North America, bringing the areas which had 

 been the sites of deposition to higher levels, faulting the rocks, and 

 affecting them by igneous intrusions. In the -western part of the 

 United States, the separation of the Triassic period from the Jurassic 

 was not pronounced, and the sedimentary history of much of this 

 part of the continent seems to have run an uninterrupted course 

 from the beginning of the Permian to the later part of the Jurassic. 

 The case may have been somewhat different north of the United 

 States, for in British Columbia and in the adjacent islands, Triassic 

 and older formations were upturned, deeply eroded, and again sub- 



