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Prof. Hayden's Explorations . 
241 
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 ; that 
from northern New Mexico to south-west Wyoming, the 
various members of the cretaceous lie in almost unbroken 
belts. 
Between the east and west there is only one great incon- 
gruity. Along the east base of the mountains the upper 
cretaceous rocks, including Nos. 4 and 5, are almost wanting, 
consisting at most of a few hundred feet of shales and lami- 
nated sandstones. Along the west base this group becomes 
a prominent and important topographical as well as geolo- 
gical feature. In the south-west, 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 south-west 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 cre- 
taceous. There is a considerable amount of arching of the 
sedimentary rocks, caused probably by the intrusion of 
wedge-like sheets of trachyte, while the broken edges of the 
beds are frequently but 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 
colouring 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 
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 their locality by their hostility. 
The work assigned this division consisted in part of a 
small area, containing about 1000 square miles, lying south 
of the Sierra la Sal. The greater portion of the work of 
this division lay north of the Grand River, limited on the 
VOL. vii. (n.s.) r 
