HAYDEN.l GEOLOGY MONUMENT CREEK GROUP. 37 



derived very largely from the granitoid rocks. The sediments become 

 finer and finer as we recede eastward from the foot of the mountains 

 into tlie plains. 



To the eastward of tbe line of the Denver and Eio Grande Railroad, 

 tbe surface is cut up into more or less rectangular masses, with rather 

 broad table-shaped summits, varying from four hundred to eight hun- 

 dred feet in height. The sides are often very steep — almost inaccessible. 

 At a remote period in the past, the erosion has been very great, carving 

 out by an almost inappreciably slow process, these broad valleys, leaving 

 these buttes here and there, composed of horizontal beds, to aid in form- 

 ing some conception of the amount of denudation which has taken place. 

 It is not possible at the present time to estimate the original thickness 

 of this group, but we believe it to have been very much greater than 

 the highest beds now existing would indicate. The summits of many 

 of these buttes are capped with a greater or less thickness of a beautiful 

 purplish trachyte, which must have ascended in the form of dikes from 

 beneath and flowed over the surface. Much of the trachyte is a sort of 

 breccia, composed of rather coarse sandstones, which must have been 

 caught in the melted material. It is quite evident that these outflows 

 occurred during the existence of the lake, though at a late period. As to 

 the real age of this group, I am inclined to regard it as Miocene, perhaps 

 Upper Miocene. The great Front or Colorado range was elevated much 

 as it is at present, though it rose some hundreds of feet during and per- 

 haps since its deposition. Some of the lower beds of the group, though 

 jutting up against the granitic mountain-sides, have evidently been 

 lifted up several hundred feet above the same strata, far east on the 

 iflains. I think it might be synchronized with the upper portion of 

 the White Eiver. group far to the northward, and is probably of the 

 same age as the fresb-water deposits in the South Park, just over 

 the range, which have yielded such an abundance of fossil leaves of 

 plants," fishes, and insects. Up to this time, the Monument Creek group 

 has yielded but few fossils, and those are vertebrates. Professor Cope 

 states that, in the summer of 1873, he made a brief examination of this 

 group for vertebrate remains, and he states that he discovered the hind 

 leg and foot of an Artiodactyle of the Oreodon type. He also has every 

 reason for believing that the fragment Magaceratops coloradoensis came 

 originally from the same locality. He further believes the group to be 

 of Miocene age, which was the conclusion of the writer in 1869. Pro- 

 fessor Cope is disposed to regard the fresh water strata in the South 

 Park as newer than Eocene and probably Miocene. 



I see no reason why they should not be of the same age as the Monu- 

 ment Creek group. The strata are horizontal, or nearly so, and hold 

 about the same position in relation to the granitic rocks in the vicinity 

 as the Monument Creek group. The sediments are quite difterent, it is 

 true, and the fossil remains most abundant and varied in character. 

 This condition might very well exist, inasmuch as we may suppose that 

 the Front range entirely s*liut off" all connection between them. Vol- 

 canic action seems to have been going on to a great extent during the 

 deposition of the South Park beds, and a great portion of the sediments 

 is composed of the eroded material of the igneous rocks. 



