56 EXPLORING EXPEDITION FROM SANTA FF, 



evidence on this point, is, however, for the most part negative. Many great ranges, 

 now supposed to belong to the Rocky Mountains, have not been visited, and of those 

 which have been examined almost none have been studied as closely as is desirable. 

 If, as the facts in our possession seem to render probable, all the ranges now grouped 

 together in the Rocky Mountain system have the same essential structure, the contem- 

 plation of this system as a whole is calculated to give us new and broader views of the 

 action of the forces by which mountain-chains have been elevated. For example, con- 

 fining our observation to our present field of inquiry, we have in Southern Utah and 

 Northern New Mexico a broad, almost unbroken mountain-belt, extending from the 

 western margin of the plains, westwardly to the western bases of the Sierra La Plata 

 and Sierra San Miguel, a distance of nearly three hundred miles. This area is, with 

 the exception of the valley of the Rio Grande, wholly occupied by mountain-ranges 

 of great elevation, and the narrow valleys which divide them. On all the older maps 

 the western half of this region is represented as occupied by the great ranges of the 

 Sierra San Juan and Sierra La Plata, to which a nearly east and west trend was 

 assigned. Our recent explorations, however, have given a new aspect to this mountain 

 group. We found the western, like the eastern portion, to be composed of a number 

 of prominent chains having a general north and south trend. The structure of these 

 western mountains is similar to that of the ranges near Santa Fe", described in the pre- 

 ceding chapter, viz : an axis of red granite, flanked on either side by strata of highly 

 inclined but almost unchanged Carboniferous limestone, containing most of our charac- 

 teristic Coal-Measure fossils ; above the limestones, entirely conformable, the red 

 beds of the Gypsum series, (Triassic); upon this, also conformable, the different mem- 

 bers of the Cretaceous formation. All these western mountain-ranges are comparatively 

 short; terminating on the north in the open area traversed by Grand River and its 

 tributaries; on the south reaching out, like so many fingers, into the plain country 

 bordering the San Juan. 



A transverse section of the Rocky Moinitains, drawn a little north of west from 

 Anton Chico across the Rio Grande, exhibits features as different from that just 

 described as could well be imagined. Just north of this line the lofty ranges of the 

 Santa Fe" Mountains fall off abruptly and disappear. West of the Rio Grande, and in 

 its immediate vicinity, lie the north and south ranges of the Valles, and the Nacimi- 

 ento Mountain; altogether perhaps twenty miles in width. West of the Nacimiento 

 the sedimentary plateau extends to and beyond the Colorado. On the line of this 

 profile, or in its immediate vicinity, the Rocky Mountain ranges have dwindled to 

 comparative insignificance. 



In a profile drawn parallel with the last, a little farther south, the Rocky Mountain 

 belt assumes far more respectable dimensions. East of the Rio Grande the Sandia 

 Mountain rises as a frowning wall 7,000 feet above the valley; 12,000 above the sea. 

 West of the river is the extinct volcano of San Mateo, to which I shall have occasion 

 to refer again, then the Sierra Madre, northwest of which, en echelon, are the chains 

 of the Sierras Tunecha and Cariso, both of which terminate south of the San Juan. 



South of this line the mountains again narrow and diminish, and the headwaters 

 of the Colorado Chiquito reach far over toward the Rio Grande. 



