6 THREE GEOLOGICAL PROVINCES. 



ming, Colorado, and Northern New Mexico, and these mountains constitute 

 a system as well denned as we may hope for geographic systems to be de- 

 fined by nature; and since my studies in that region in 1867 '68, I have 

 been accustomed to speak of them as the Park Mountains. The system is 

 composed of many ranges, some w r ell defined, others complex^inclosing, or 

 nearly so, the North, Middle, South, and San Luis Parks, with many minor 

 valleys and parks; and there are short outlying spurs and ranges with other 

 parks and valleys. The principal mountain ranges are composed of nieta- 

 morphic rocks with unaltered sedimentary beds on their flanks. Some of 

 the more western mountains are chiefly of this latter material, and many 

 of the subsidiary mountains are of eruptive origin. Altogether they con- 

 stitute a geological province characterized by a great development of meta- 

 morphic crystalline schists with patches and structural basins of marine and 

 lacustrine sediments, and a complicated series of vulcanic formations. 



Southward from the Oregon line, through Western Utah, Nevada, 

 Southeastern California, and perhaps across the Colorado River in Western 

 Arizona, many short and more or less distinct north and south ranges are 

 found. The valleys and plains separating these ranges are covered with 

 rather late subaerial gravels masking the underlying formations. The 

 ranges are composed of metamorphic crystalline schists with Paleozoic beds 

 on their flanks, or sometimes, even in large part of Paleozoic materials both 

 complicated to a greater or less extent with eruptive beds; these eruptive 

 beds themselves sometimes forming the principal component parts of the 

 ranges. 



The little streams that have their simrces in these mountains empty 

 into salt lakes, or elsewhere their waters are lost in the sands; as it is popu- 

 larly said they disappear in sinks. The most important of these is the 

 Great Salt Lake, but there are many other basins without drainage to the 

 sea. A few of the ranges are drained into the Colorado River. To this 

 group Mr. G. K. Gilbert has given the name Basin Range System, which 

 seems appropriate. 



It will be found convenient also to treat the area occupied by this group 

 as a distinct geological province. It has a series of sedimentary beds differ- 

 ing widely from the Plateau Province; they are older, and the sediments of 



