Scien iij •'(■ Intelligence. 



i the gigantic hog-back of the 



-hwest, he visited the Sierra Abajo, a small 

 group of mountains which lie in [vision. Rah. 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 .virion. 

 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 rocks, caused probablv bv the intru- 

 sion of wedge-like sheets of" trachyte, while the broken edges of 

 the beds are frequently hem abruptly 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 



He states that the northern limit of ancient eliff builders in 

 Colorado and Eastern ('tab is hanUv above latitude 37° 45'. 



The Grand River Division was directed by Henry Gannett, 

 topographer, with Dr. A. C. Peale as geologist. James Stevenson, 

 executive officer of the survey, accompanied this division for the 

 purpose of assisting in the management of the -Indians, who last 

 year prevented the completion of the work in this locality by their 



The work assigned this division consisted in part of a small 

 area containing' about ]. 1 1( in s,p-,a : the Sierra 



la Sal. The greater portion of the work of this division lav north 

 of the Crand River, limited on the north bv the parallel of 3:i 3 W 

 aud included between the meridians of 108° On' and 109° 30'. 



The geological work of this division, by Dr. Peale, connects 

 directly with that done by him in 1874 and 1875. Sedimentary 

 formations prevail in both districts visited during the past season. 



The country first examined lies between the' San Miguel and 

 Dolores Rivers, extending northward and northwestward from 

 Lone Cone Mountain. The general character of this region is 

 that of a plateau cut by deep gorges or canons, some of which. 

 especially toward the north, extend from the sandstones of the 

 Dakota group to the top of the Red Beds. The depth of the 



for away from the main streams thev are drv the greater portion 

 of the vear. There are no great disturbance's of the strata., what 

 folds do occur being broad and comparatively gentle. 



The San Miguel. River leaving tin San Juan Mountains flows 

 toward the northwest and, with its tributaries, cuts through the 

 ... exposing the variegated beds 

 lying beneath that have general h b . referred > tl Jurassu 

 About twenty-five or thirty mile's north of Lone Cone, the river 

 ""- i abruptly to the west an 



