MONOCLINAL RIDGES. 23 



GEOGRAPHIC STRUCTURE GF THE BASIN PROVINCE. 



. 



In this province that orographic type which I have described as the 

 Basin Range structure prevails. 



In the consideration of the structure of these ridge like mountains, it is 

 necessary to distinguish clearly the two more important elements involved, 

 viz, that of the metamorphic and unaltered sedimentary formations, and 

 that of the eruptive beds. 



The former appear in simple monoclinal ridges of displacement, but 

 the extravasated material may occupy any position in relation to the simple 

 ridges ; sometimes it is found appearing on the flanks, sometimes burying 

 portions of the ranges, sometimes extending in subequal masses in trans- 

 verse or oblique directions to the ridges proper, and in many ways compli- 

 cating the topographic structure. It is of the structure of the monoclinal 

 ridges only that I now speak. These ridges are not residuary fragments of 

 anticlinal flexures eroded in intaylio, for wherever the structure at the foot 

 of the escarpment is not concealed by subaerial gravels, the beds .seen at 

 the summit of the ridge, or known to belong to a still higher horizon, appear 

 again at the foot of the escarped face, showing that they have been thrown 

 to that position by a fault. The ridges themselves occupy the place of max- 

 imum upheaval. In the summer of 1870 I had some opportunity to examine 

 a few of these ridges while oil a trip from Salt Lake City to Fillmore, Bea- 

 ver, and Saint George, in Utah. In the winter of 1871-72, I spent a few 

 weeks studying the mountains west of the Rio Virgen, and again in 1873 

 while engaged in prosecuting some ethnographic studies I visited many 

 points in Western Utah, Nevada, and Southern California, making cursory 

 examinations of mountain structure on my way; but Mr. G. K. Gilbert, 

 while engaged as geologist of the Wheeler expedition, made a much more 

 thorough study of this region. In his report of the geology of that region 

 for 1872, and published in 1874, page 50, under the head of "Mountain 

 Building/' Mr. Gilbert presents a "diagram of generalized mountain sections 

 discounting denudation," which I reproduce (Fig. 7), preserving his lettering. 



