T6 Explorations in Colorado. [ February, 
In the southwest, where it forms the “‘ mesa verde” and the cap 
of the Dolores plateau, it comprises upwards of two thousand 
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 Kbejos 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 other four 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 rocks, probably caused by the in- 
trusion of wedge-like sheets of trachyte, while the broken edges 
of the beds are frequently bent abruptly 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 re- — 
gion 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 River Division was directed by Henry Gannett, 
topographer, with Dr. A. C. Peale as geologist. James Steven- 
son, 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 local- 
ity by their hostility. 
The work assigned this division consisted in part of a small 
area, containing about one thousand square miles, lying south of 
the Sierra la Sal. The greater portion of the work of this di- 
vision lay north of the Grand River, limited on the north by the 
ae of =å 30’ and included beirom the meridians of 108° 
and 109° 
This es took the field at Cañon City, Colorado, about the 
middle of August. The party traveled nearly west, up the Ar- 
kansas River, over Marshall’s Pass and down the Pomichi and 
Gunnison rivers to the Uncompahgre (Ute) Indian agency. 
Here they secured the services of several Indians as escort in the 
somewhat dangerous country which they were first to survey. 
This area lying south of the Sierra la Sal was worked without 
difficulty. It is a broken plateau country and presents many 
curious pieces of topography. Eleven days were occupied in this 
work. 
