00 RICHTHOFEN NATURAL SYSTEM 



accumulation of Palteozoic and Mesozoic sediments, and the subsequent denudation of 

 these, though grand in the extreme, has only been sufficient to expose to view the 

 ancient granite in some localities, among which we mentioned that on the Colorado 

 River. Other masses of ancient granite are visible at the surface in numerous places in 

 the Great Basin, but their exact relations still remain to be determined, while in the 

 Rocky Mountains they are one of the prominent features, partaking largely in their 

 structure. 



The next period of interest, in regard to the occurrence of eruptive rocks, is that 

 of the deposition of Triassic and Liassic sediments in great aggregate thickness, which 

 were found by Whitney to extend from the Pacific coast far into the Great Basin. They 

 prove that all this country was then still submerged beneath the sea, while the Rocky 

 Mountains formed probably a broad belt elevated above it. It is in this period that 

 were ejected the quartzose porphyries of the County of Plumas, in northern California, 

 almost contemporaneously with those of the southern Alps. Whether this event, which 

 was probably not limited to the region mentioned, was attended by any changes in the 

 configuration of the surface, cannot yet be decided. After it, however, they must have 

 taken place on a grand scale, reminding one of the emergence of the central body of the 

 Alps from the sea after the deposition of the iufra-Liassic limestones. The ejection of 

 granite found, it appears, almost the whole country embraced between the western 

 slope of the Sierra Nevada and the eastern slope of the Rocky Mountains lifted out of 

 the sea. But changes greater than those which had preceded them, appear to have 

 attended and followed those gigantic outbreaks. They were probably due, in a great 

 measure, to the intense metamorphism which was connected with them, and has com- 

 pletely changed the petrographical character of the preceding sedimentary deposits. 

 When the volcanic era commenced, which was probably in the Miocene epoch, all the 

 ancient formations of the Sierra Nevada were nearly turned upon their edges, and the 

 depressions of the surface in the Great Basin were filled with saltwater. But the alti- 

 tude of the entire plateau above the level of the sea was probably insignificant at that 

 time compared with what it is now, as may be inferred from the fact, that rivers did then 

 flow on the present western slope of the Sierra Nevada, parallel to its crest, which they 

 could not have done if that slope had had its present inclination. We must imagine 

 that where the great mountain range raises now its lofty summits, a hilly country ex- 

 tended then, ascending slightly to the east. The first outbreaks of volcanic rocks found 

 those rivers still flowing in their beds, as is proved by the higher sediments in the old 

 river-channels, which consist of volcanic tufas. But great changes occurred after the first 

 commencement of the volcanic era, changes which contributed probably more towards 

 imparting to the western portion of North America its present features than any which 

 had preceded them. The volcanic belt extending along the entire western coast of 

 America, had its greatest breadth between the coast of California and the Rocky Moun- 

 tains, and the eruptive activity has been violent over that vast region. It appears that 

 to that era is due the principal part of the elevation of the high table-lands. Such, at 

 least, is the case in their western portion. The crest of the Sierra Nevada must have 

 been elevated at a quicker rate than its western foot, as distinc traces are left in the 

 gravel deposits that those ancient rivers which were flowing parallel to its crest have 

 d-.'l, 



