686 GEOLOGY 



eastern Idaho, and the sea probably reached this region by way of 

 Utah, Nevada, and Southern California. Later formations of the 

 system are not found east of western Nevada and eastern Oregon. 1 

 The Triassic system is found as far north as Alaska. 



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, to counterbalance the waste, 

 unless the great thickness is due to oblique deposition. 



The succession of faunas in the Trias of the Pacific coast in- 

 dicates that considerable changes in the physical geography of the 

 northern Pacific were in progress during the period. These will 

 be referred to in connection with the faunas of the system. 



Climatic Conditions 



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

 not only of America but of Europe, is good evidence of wide-spread 

 aridity. The prevalent redness of the system, in other continents 

 as well as our own, is also commonly regarded as an indication of 

 aridity. Some of the peculiarities of the Newark conglomerate 

 also might find their explanation in aridity. In an arid climate, 

 expansion and contraction due to changes of temperature are so 

 great as to be effective in disrupting 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 often characterize arid climates, might account for the trans- 

 portation of debris from the place of its origin to the place of its 

 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 con- 

 glomerate, their rounding and wear would have to be attributed 

 to the waves of the body of water in which deposition took 

 place. 



1 Stanton, Jour. Geol., Vol. XVII. 



