LETTER OF THE OEOLOGIST. XV 



they proceeded to the great volcanic plateau at the head of White River. 

 The final station was made between the White and Yampah Eivers, in 

 the northwestern corner of Colorado. During this brief season Mr. 

 Wilson finished about one thousand square miles of topography, and 

 made eleven primary geodetic stations, thus connecting together by a 

 system of primary triangles the whole of Southern and Western Colo- 

 rado. 



In company with the triangulation party, Mr. Holmes made a hurried 

 trip through Colorado, touching also portions of ISIew Mexico and Utah. 

 He was unable to pay much attention to detailed work, but had an ex- 

 cellent opportunity of taking a general view of the two great plain belts 

 that lie, the one along the east, the other along the west base of the 

 Eocky Mountains. For nearly two thousand miles travel he had 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 persistency of the various 

 groups of rocks throughout the east, west, and north, and especially in 

 the west 5 that from Northern New Mexico to Southwest Wyoming the 

 various members of the Cretaceous lie in almost unbroken belts. 



Between the east and the west there is only one great incongruity. 

 Along the east base of the mountains the Upper Cretaceous recks, in- 

 cluding 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 

 geological feature. In the southwest, where it forms the " Mesa Verde " 

 and the cap of the Dolores Plateau, it comprises upward of two thou- 

 sand feet of coal-bearing strata, chiefiy sandstone, while in the north it 

 reaches a thickness of 3,500 feet, and forms the gigantic "hog-back" of 

 the Grand Eiver 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 identical 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 Cretaceous. There 

 is a considerable amount of arching of the sedimentary rocke, caused 

 probably by the intrusion of wedge-like sheets of trachyte, while the 

 broken edges of the beds are frequently, but abruptly, pressed up, as if by 

 the upward or lateral pressure of the rising mass. He was able to make 

 many additional observations on the geology of the San Juan region, 

 and secured much valuable material for the coloring of the final map. 



He states that the northern limit of ancient cliff builders in Colorado 

 and Eastern Utah is hardly above latitude 37^ 45'. 



The Grand Eiver division was directed by Henry Gannett, topogra- 

 pher, with Dr. A. C. Peale as geologist. James Stevenson, executive 

 officer of the survey, accompanied this division for the purpose of assist- 



