TO JUNCTION OF (J RAN I) AND GREEN RIVERS. 55 



and I tab — naturally fall into two great groups: first, that of the mountain ranges; 

 second, that oi the great plateau which they inclose. For convenience, these will be 

 considered separately. 



MOUNTAINS SURROUNDING THE COLORADO BASIN. 



Nothing like a thorough exploration of the numerous and complicated mountain 



systems of the region under consideration has as yet been made, and years of patient 

 study must be devoted to the subject before their intimate structure or relations to each 

 other will be fully comprehended. Only a skeleton sketch of their extent, structure, 

 and direction can now be honestly given, and the inferences drawn from the facts 

 already observed will be liable to considerable modification in the light of future ex- 

 plorations. 



The mountain chains and groups under consideration are as follows: 



1. 'The Llocky Mountain system. 



2. The ulogollon Mountains. 



.'). (Vrhat and Aquarius ranges. 



1. The mountains of Lower Colorado (Black and Mohave Mountains, &c). 



5. 'The Wasatch Mountains. 



6. The San Francisco group (latch- an active volcano). 



7. The Mount Taylor (San Mateo) (lately an active volcano). 



8. The Sierra Tucane (isolated mountain). 



9. The Sierra Ahajo (isolated mountain). 



lv). The Sierra La Sal (isolated mountain). 



11. The Sierra La Late (isolated mountain). 



By reference to the accompanying map it will he seen that the vast moiintain-helt 



of the Rocky Mountain system, the great backbone of our continent, forms the eastern 

 boundary of the Colorado basin, and, by its drainage, supplies hv far the greater part 

 of the flow of that stream. In the deep sinuosities of outline, occasioned by the inter- 

 ruption of many of the ranges of this great system, the courses of the San Juan, Grand 

 River, and Little Colorado are laid. Between the Rocky Mountains on the east, and 

 the Wasatch, Aquarius, and Cerbat ranges on the west, the Colorado plateau lies as 

 a comparatively level plain, its northern and southern boundaries less accurately 

 defined, and in part as vet unknown. 



ROOKY MOUNTAIN SYSTEM. 



In the preceding chapter and in another report, I have given my own observa- 

 tions on portions of this mountain system, and have expressed my views upon the 

 indefinite and shadowy nature of the classification, according to which all portions 

 of the immense and irregular mountain-holt which traverses the central portion 

 of our continent are included in a single name and referred to a single epoch in 

 geological history. So far as the limited explorations by geologists have extended, 

 it is true that a, general uniformity of composition and trend has been observed, 

 and, as will he seen in the subsequent pages, I have now no new facts to report 



which indicate the necessity of its separation into several distinct systems. 'The 



