Mesozoic and Ccenozoic Geology and Paloiontology. 251 



gravels in valle\'s and over plains where mountains rise to higher alti- 

 tudes on either side, and having in man}^ cases actual)}^ 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 cla}', accumulations of sand, of gravels, and bowlders, having, 

 in a general way, all the lithologic characteristics of "drift," are very 

 common in the Rock^^ 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 from north to south, and 50 miles from 

 east to west, or about 2,000 square miles, called by Dr. Ha^'den, 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 lower 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, whita and yellow, 

 varj'ing 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 the}^ recede eastward 

 from the foot of the mountains into the plains. 



* U. S. Geo. and Geogr. Sur. of Colorado and Adjacent Territorj-. 



