406 GEOLOGY OF THE DEXVEK BASIN. 



THE CONDITIONS OF COMPOSITION AND STRATIGRAPHY IN THE DENVER BASIN. 



WATER-BEARING HORIZONS. 



The water-bearing sandstones underlying the Denver Basin at depths 

 within economic reach are: That capping the Fox Hills formation; the 

 basal member of the Laramie: the conglomerates and coarse sands at the 

 bottom of the Arapahoe; a broad arenaceous zone midway in this forma- 

 tion; and several sandy layers distributed throughout the shales of these 

 formations and of the overlying Denver beds. Beneath this series the 

 water-bearing strata lie at great depths: the first several thousand feet 

 down in the Montana clays; a second — the Dakota sandstones — between 

 l.ODO and 2,000 feet lower; and, finally, the series of sandstones and 

 conglomerates of the Trias. 



The sandstones which occur in the clays of the Laramie, Arapahoe, 

 and Denver formations arc uncertain. They generally assume a lenticular 

 form, although perhaps attaining considerable areal extent. They enhance 

 the value of the formation in which they occur as a water-bearing series, 

 however, by their recognized liability to occur at any horizon. In the 

 Arapahoe their importance in this respect has become well established; in 

 the Laramie, such bodies are of less frequent occurrence. 



The several water-bearing sandstones differ in composition and texture 

 one from another and also within themselves from point to point, but those 

 of most uniform distribution are also the most uniform in texture. 



Fox Hiiis. — The Fox Hills sandstone, which occurs at the summit of the 

 formation, is composed chiefly of fine quartz grains, with a little mica. It 

 is somewhat ferruginous, of greenish-yellow color and even texture. It 

 passes by gradual transition to the shales below, which become less and less 

 arenaceous in depth and finally take on all the characteristics of the great 

 body of Montana clays. 



Lower Laramie. — The sandstones of the lower Laramie are of much 

 coarser grain than the Fox Hills, are white or light-gray in color, but 

 slightly ferruginous, are open in texture, and occur in heavy benches, the 

 lowest two either in a single body or separated by a narrow band of coal 

 or carbonaceous shale. These last are, together, 120 feet, and with the 



