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Mesozoic and Cenozoic Geology and Paleontology. 109 
gravels in valleys and over plains where mountains rise to higher alti- 
tudes on either side, and having in many cases actually seen the cliffs 
breaking down, and the gravels rolling out on the floods of a storm, I 
am not willing to disregard explanations so obvious, and so certain, for 
an extraordinary and more violent hypothesis. Irregular accumula- 
tions of clay, accumulations of sand, of gravels, and bowlders, having, 
in a general way, all the lithologic characteristics of “drift,” are very 
common in the Rocky mountain region, and in many cases their origin 
can be traced to ordinary atmospheric agencies acting on the adjacent 
hills and mountains; and no glaciers or icebergs are needed for their 
explanation. 
We learn from Dr. Hayden,* that on the high divide between the 
drainage of the Arkansas and South Platte rivers, there occur fresh- 
water lake deposits, having a thickness of 1,000 or 1,500 feet, and cov- 
ering an area of about 40 miles trom north to south, and 50 miles from 
east to west, or about 2,000 square miles, called by Dr. Hayden, in 
1869, the “ Monument Creek Group,” from the fact that the atmo- 
spheric agents have carved out of the beds peculiar monuments or col- 
umns. He referred the deposits to Miocene or Pliocene age ; later, in 
1873, Prof. Cope, from the evidence of the hind leg and foot of an Ar- 
tiodactyle, and a fragment of Megaceratops coloradoensis, referred the 
deposits on the Colorado divide, perhaps the same, to the age of the 
Miocene. The texture of the rocks is quite varied. 
The lowet portion is composed of rather massive beds of sand- 
stone, varying from a pudding-stone to a fine-grained sandstone, usu- 
ally of a light color, sometimes of a yellow or iron-rust, with their in- 
tercalations of arenaceous clay. In the distance, the whole group, in 
many localities, presents a chalky-white appearance. At the im- 
mediate base of the mountains, just south of the small lake on the di- 
vide, the rocks are variegated sandstones, brick-red, white and yellow, 
varying in texture from a fine sandstone to a pudding-stone, with all 
the signs of deposition in moving waters. Still farther north, on the 
divide proper, the beds jut against the granites, inclining not more 
than 3°, and are made up of a coarse aggregate of feldspar and quartz 
crystals, so that it resembles a very coarse granite. It is plain that the 
sediments of this group were derived very largely from the granitoid 
rocks. The sediments become finer and finer as they recede eastward 
from the foot of the mountains into the plains. 
* U.S. Geo. and Geogr. Sur. of Colorado and Adjacent Territory. 
