F. GEOGRAPHY. 245 



New Mexico and Utali. He was unable to pay much atten- 

 tion to detailed work, but bad an excellent opportunity of 

 taking a general view of tbe two great plain belts tbat lie 

 tbe one alons: the east, the other alons: the west base of the 



Cj / --7 



Rocky Mountains. For nearly 2000 miles' travel he liad con- 

 stantly in view the cretaceous and tertiary formations, among 

 which are involved some of the most interesting geological 

 questions. He observed, among other things, the great per- 

 sistency of the various groups of rocks throughout the east, 

 west, and north, and especially, in the west, that from North- 

 ern New Mexico to Southern Wyoming the various members 

 of the cretaceous lie in almost unbroken belts, while the ter- 

 tiaries are hardly less easily followed. 



Between the east and the west there is only one great in- 

 congruity. Along the east base of the mountains the up- 

 per Cretaceous rocks, including Nos. 4 and 5, are almost 

 wanting, consisting at most of a few hundred feet of shales 

 and laminated sandstones. Along the west base this group 

 becomes a prominent and important topographical as well 

 as freoloo-ical feature. In the northwest, where it forms 

 the ''Mesa Verde," and the cap of the Dolores plateau, it 

 comprises upward of 2000 feet of coal-bearing strata, chiefly 

 sandstone, while in the north it reaches a thickness of 3500 

 feet, and forms the gigantic hog-back of the Grand River 

 Valley. 



While in the southwest he visited the Sierra Abajo, a small 

 group of mountains which lie in Eastern Utah, and found, as 

 he had previously surmised, that the structure was identi- 

 cal with that of the four other isolated groups that lie in 

 the same region. A mass of trachyte has been forced up 

 through fissures in the sedimentary rocks, and now rests 

 chiefly upon the sandstones and shales of the lower Creta- 

 ceous. There is a considerable amount of arching of the sed- 

 imentary rocks, caused probably by the intrusion of wedge- 

 like sheets of trachyte, while the broken edges of the beds are 

 frequently bent abruptly up as if by the upward or lateral 

 pressure of the rising mass. He was able to make many ad- 

 ditional observations on the geology of the San Juan region, 

 and secured much valuable material for the colorimx of the 

 final map. 



He states that the northern limit of the ancient clifl" build- 



