58 EXPLORING EXPEDITION FROM SANTA FE 



deposited during a period in which a subsidence of several thousand feet took place. 

 This is shown by the immense extent of the Lower Cretaceous sandstones, a littoral 

 formation, the immediate debris of the land, containing only and abundantly the remains 

 of land-plants; these sandstones being overlaid by a great thickness of marine strata. 



5. At the close of the Cretaceous age a period of elevation began, which continued 

 to the Drift epoch, when the continent was again depressed, to be again elevated, 

 though not having as yet reached its former level. 



The facts upon which these conclusions are based are briefly these: (.) The 

 Tertiary beds of the country bordering the Rocky Mountains on the east are mainly 

 fresh-water deposits. (?;.) They were precipitated upon surfaces which had suffered 

 enormous subaerial erosion, filling troughs and basins excavated in the Cretaceous or 

 Gypsum formations; the Tertiary strata being unconformable. (c.) The investigations 

 of Messrs. Meek and Hayden have shown that in these Tertiary basins the lower beds 

 sometimes contain estuary shells, the upper exclusively fresh-water species. 



6. We may fairly infer from the facts given above that the great elevatory move- 

 ment, which gave to the Rocky Mountain region the character it now exhibits, took 

 place between the close of the Cretaceous period and that of the Miocene Tertiary. 

 To this general rule, however, we cannot yet say that there are' not marked exceptions. 



MOGOLLON MOUNTAINS. 



Perhaps no important moiintain chains within the territory of the United States 

 are so little known as these. No geologist has ever visited the region which they 

 occupy, and I am not aware that any record has been made of the observations 

 of the Army officers or others who have traversed it. In his synopsis of the 

 mountains of North America, M. Marcou has referred the Mogollon ranges to a 

 distinct system, and has assigned to them a trend very different from those of the 

 mountains nearest them on either side. This classification has been, however, advanced 

 on grounds wholly inswfficient, and may be considered as purely imaginary. As I 

 have stated in my former report, my own convictions are that future explorations will 

 show that the ranges of the Mogollon and Sierra Blanca have a north and south or 

 northwest and southeast trend, and are to be grouped with the Rocky Mountain ranges 

 on the east, or those of the Sierra Nevada system on the west. My reasons for this 

 belief are, that verbal statements of parties who have visited these mountains indicate 

 their structure to be essentially that of the Rocky Mountains; i. e., axes of red or 

 grayish granite, flanked by Carboniferous limestones with their characteristic fossils; 

 some of which, such as Productus semireticulatus, have been brought in. In addition to 

 this, it may be said that ranges of mountains, with an east and west trend, would be a 

 strange anomaly in the topography of New Mexico. Such a thing I have not yet met 

 with in any portion of the central or western parts of our continent which I have visited. 



MOUNTAINS OF THE LOWEK COLORADO. 



These ranges, which form the southwestern boundary of the table-lands, seem 

 but continuations of the metalliferous chains of Sonora, passing with a northwest 

 trend up into Nevada and California, there blending with the Sierra Nevada. In trend 



