ELK MOUi^TAIN REGION. 151 



Silica 9.74 



Alumina - . 5.57 



Ferric oxide 1.93 



Ferrous oxide 38.67 



Manganous oxide. -• 2.38 



Lime 7.64- 



Magnesia 1.20 



Soda and potassa 0.46 



Phosphoric acid . . — trace 



Carbonic acid - - - . 32.04 



Water - • trace 



Organic matter trace 



99.63 

 Although these clay ironstones of the Colorado Cretaceous have as 

 yet nowhere been observed in extensive deposits, it is by no means impos- 

 sible that they may be found near the railroad in close proximity to limestone 

 bodies and lignite beds of the Laramie formation; in which case they 

 may at some future time prove to be of practical value. One of the most 

 marked structural features in the region of Elk Mountain is seen in the 

 prominent ridges of Fox Hill sandstone, which lie just outside the eroded 

 valleys of the Colorado Cretaceous. In many localities, the Colorado 

 clays fall away in dip, and the overlying sandstones pass out upon the 

 plain in low rolls with diminished dips. Here, however, along the face of 

 the mountain, the Fox Hill presents very persistent ridges, with mural 

 faces toward the range, and dipping at high angles. To the eastward of 

 Elk Mountain, along the north side of the Medicine Bow River, these ridges 

 extend in an east and west line. North of Scotch Creek, the beds are con- 

 siderably broken up, and Simpson Ridge, a long anticlinal fold, trends 

 off to the northward. West of Simpson Ridge, the west side of this fold 

 curves round, striking east and west, until north of Rattlesnake Creek, 

 when it bends off to the northwest in the direction of Saint Mary's Peak. 



A recorded strike of the main ridge, north of Elk Mountain, indicates 

 north 35° to 40° east, with a dip of 52° to 57° northwest. At the southern 

 base of the ridge, the Fort Pierre beds pass up into brownish sandstones, over- 

 laid by gray beds, and in turn passing up into others of a rusty-brown 



