REPORT OP' THE SECRETARY. 53 



The final station was made between the White and Yanipah Rivers, 

 in the northwestern corner of Colorado. In the two months he was in 

 the field Mr. Wilson traveled 1,400 square miles, finished about 1,000 

 square miles of topography, and also made eleven geodetic stations, 

 connecting by a system of primary triangles the whole of Southern 

 and Western Colorado with the triangulation of the previous years. 

 Mr. W. H. Holmes accompanied the triangulation -party as geologist. 

 From the rapidity with which they were obliged to travel he was unable 

 to do much detailed work, but he had an excellent opportunity to cor- 

 relate the formations of the plains east of the mountains with those of 

 the pleateau region on the west. As he had previously surmised, he 

 found the structure of the Sierra Abajo to be identical with that of the 

 other isolated mountain-groups to the south and east of the Abajo in 

 in his district of 1875. A mass of trachyte has been forced up through 

 fissures in the sedimentary rocks, and now rests chiefly upon the sand- 

 stones and shales of the Lower Cretaceous. The sedimentary rocks are 

 arched from the intrusion of wedge-like sheets of trachyte, and the bro- 

 ken ends of the beds are frequently bent upward, as if by the upward 

 pressure of the igneous mass. The division left the field about the 12th 

 of October at Rawlins Springs, iu Wyoming Territory. 



The Grand River division was in charge of Henry Gannett, topog- 

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

 officer of the survey, also accompanied the party a portion of the season. 

 The party took the field at Caiion City, August 23, and marched west- 

 ward to the Uncompahgre agency, a few miles east of the lower dis- 

 trict assigned to the division. The country assigned to it for explora- 

 tion consisted of two detached portions, one of about 1,000 square miles, 

 lying south of the Sierra la Sal and east of the Sierra Abajo, extending 

 eastward to the Rio San Miguel and southward as far as the latitude of 

 Lone Cone. The other lay north of the Grand River, extending from 

 the river to the crest of the Roan or Book Cliff's. Its eastern limit was 

 the meridian of 108° and the western 109° 30', comprising about 3,000 

 square miles. 



Work was commenced in the southern district early in September. 

 The country surveyed is entirely made up of broken plateaus, almost 

 without water, except in the Dolores and San Miguel Rivers, and sev- 

 eral smaller streams. It has very little agricultural value, but portions 

 of it would make fair winter- ranges for stock. The region, however, is 

 geologically interesting, on account of several folds which, with the 

 subsequent erosion, have beautifully exposed the several formations. 

 Coal occurs at several places in the Cretaceous rocks, and on the Uucom- 

 pahgre River, a few miles above the agency, there is semi-anthracite 

 coal. Gold placers have been discovered on the San Miguel River, 

 and in the mountains south of the district prospecting has been vigor- 

 ously prosecuted. This portion of the San Juan mining region bids fair 

 to become one of the most important and successful districts, as indi- 

 cated by the developments already made. 



