92 GEOLOGICAL SURVEY OF THE TEREITOEIES. 



near the foot of Elk Mountain. This is quite a large stream, with clear 

 pure water, fringed with a wide belt of bitter cotton-wood. 



Elk Mountain forms a short range, with the highest point facing the 

 river, and resembles the short range with abrupt front east of the Little 

 Laramie Eiver. 



The metamorphic rocks have been elevated, while the unchanged Ter- 

 tiary beds jut up against the base without the usual appearance of a 

 series of upheaved ridges, as we find in approaching the nucleus of the 

 mountains. This range is only 10 or 20 miles long. It forms what I 

 have called an abrupt anticlinal — that is, on one side all the rocks seem 

 to have been dropped down at the base and the mountain-side, present- 

 ing an almost vertical escarpment, while the opposite side slopes gently 

 down, revealing the upturned edges of all the unchanged rocks in the 

 region, reposing upon, or inclining at, moderate angles from the meta- 

 morphic rocks. 



The numerous branches which constitute the sources of the Medicine 

 Bow Eiver form a broad valley scooped out, evidently, from the yielding 

 rocks, so that Elk Mountain is to a certain extent an isolated range. 

 The Tertiary beds dip away from the foot of the mountain' northward, 

 and passing across the ridge we find them composed of a series of brown 

 and dark-brown indurated clays and sands, with layers of more or less 

 laminated rusty sandstone, very fine, but with a little lime and a strong 

 tendency to a concretionary structure, varying in thickness from 2 feet 

 to 10 or 12. 



Sometimes these rocky layers swell out to a considerable thickness, 

 then again diminish until they are lost in the loams, sands, and clays. 

 They usually protect the ridges from wearing down and show more dis- 

 tinctly the dip of the beds, which here is 30° to 40°, about 20° west of 

 north. Elk Mountain seems to incline about northwest and to face 

 southeast, the southeast front being abrupt, while the northwest slopes 

 gently down so as to show clearly that x)ortion of the anticlinal. 



In the Tertiary ridges just described are quite extensive beds of lig- 

 nite. The first ridge, near the Medicine Bow stage-station, has a bed of 

 coal six feet thick, and the harder layers above and below the coal are 

 filled with indistinct vegetable impressions. 



The interval between the first main ridge and the second is about one 

 and a half miles, and in that interval are shown several beds of lignite 

 and layers of light-gray fine-grained siliceous rocks. 



The second main ridge inclines three to five degrees, and this is com- 

 posed of a variety of beds, the general color being brown or light-drab, 

 while the harder layers are rusty sandstone. 



One bed, perhaps 50 feet in thickness, is composed of fine gray indu- 

 rated sand with a greenish tinge. At the summit of this ridge is a layer 

 of melted or baked rocks, caused by the burning out of the coal-beds 

 beneath. Impressions of deciduous leaves are found here in consider- 

 able numbers. Some of the harder layers are composed of an aggre- 

 gate of the crystals of feldspar and quartz, as if the sediments were 

 derived directly from the disintegration of the metamorphic rocks. 



The concretionary rocks break in pieces in a variety of ways ; some of 

 them exfoliate, as it were — that is, they are formed of concentric coats 

 which fall off from the nucleus ; others seem to split in thin laminte like 

 cutting an apple into thin slices ; others break into irregular fragments. 

 All exhibit the same rusty-yellow color on exposure. This is doubtless 

 due to the decomposition of the sulphuret of iron, which seems to be to 

 a greater or less extent in all the rocks. On coming in contact with 

 the atmosphere or moisture this sulphuret of iron becomes the oxide of 



