TO JUNCTION OF GRAND AND GREEN RIVERS. 57 



Still further south, on and below the parallel of 34, an almost unbroken succes- 

 sion of mountains stretch from the Sierra Blanca, the Sacramento, and Guadalupe 

 Mountains, all the way across to the Colorado. Of these, those lying near the Rio 

 Grande, both east and west, have apparently a structure similar to that of the ranges of 

 the Rocky Mountains further north. West of these an immense area is occupied by 

 the Mogollon Sierras, as yet almost unknown. Still further west are the ranges cross- 

 ing the Lower Colorado, and belong-ing to the system of the Sierra Nevada. What is 

 the relative space occupied by these different mountain groups; whether they repre- 

 sent one, two, or three distinct systems; and if distinct, what are their relative ages, 

 are questions which future observations alone can answer. 



In regard to the precise age of the Rocky Mountains, as before stated, we want 

 more extended and minute observations before we can determine whether the elevation 

 of all parts of the system was synchronous, and whether, if so, it is to be referred to 

 a single epoch, or was continued through several geological periods. There are, how- 

 ever, some facts which lead me to conclude that the elevation of the different ranges 

 now referred to this system was not in all cases synchronous, and that they are the 

 result of the action of forces operating through all geological time, from the earliest 

 Paleozoic period to the- present. Still further as a corollary to the last proposition 

 the Rocky Mountain region has suffered several alternations of elevation and depression. 

 Much of the evidence on this subject is given in my report to Lieutenant Ives, page 

 . It is briefly as follows : 



1. The Paleozoic strata, several thousand feet in thickness, which underlie the Car- 

 boniferous formation in the great canon of the Colorado, are wholly wanting on the sides 

 and summits of the mountain ranges which bound the Colorado basin, both on the east 

 and west Cerbat and Aquarius ranges, Santa F^ and Nacimiento Mountains, Sierras 

 La Plata, Cariso, &c. On these ranges the Carboniferous strata, very little metamor- 

 phosed, rest directly upon the granite ; the natural inference from these facts being that 

 the mountains enumerated above existed, at least in embryo, previous to the deposi- 

 tion of the older Paleozoic rocks, and were elevated above the ocean from which the 

 lower strata of the Colorado section were deposited. 



2. The explorations of Dr. Hayden have demonstrated that the granitic axis of 

 the Black Hills is flanked by the Potsdam sandstone, now exposed by upheaval, and 

 overlaid by the Carboniferous and more recent strata; the evidence being conclusive 

 that this portion of the Rocky Mountain system was beneath the waters of the primeval 

 ocean. 



3. In the fresh lava-streams of the Raton Mountains, the Valles, in the Mal- 

 pais near Fort Stanton, and about San Mateo, as well as in various other portions of 

 the Rocky Mountain region, we have evidence that most violent volcanic action has 

 been in operation distinctly within the present epoch, having been continued from the 

 Middle Tertiary period, if not dating back still further. 



4. As stated in the descriptions of the Arkansas Tertiary basin and the geology 

 of the region about Cottonwood Spring, (Chapter I,) we have satisfactory proof that, 

 previous to the deposition of the Lower Cretaceous strata, the central portion of the 

 continent was above the ocean level, and that the Cretaceous sediments were mainly 



8 s F 



