so THE GEOLOGICAL STRUCTURE, ETC. 



It is foiind that, as is the case in the Atlantic and Gulf 

 Border regions, the great bulk of the artesian water is derived 

 from the sandy beds lying at the base of the Cretaceous 

 formation. The overlying Tertiary beds also yield generous 

 supplies of sub -artesian water. 



In the centre of the North American continent, the Upper 

 Cretaceous alone is represented. The formation has been 

 subdivided into the following four groups, in ascending order : — 

 Dakota Sandstones, Colorado, Montana, and Laramie Beds. 



The Dakota Sandstones lying at the base are, with minor 

 exceptions, extremely well adapted for the absorption and 

 transmission of large quantities of water ; and, in addition, the 

 beds attain in places considerable thickness. In New Mexico, 

 they measure 1,700ft. Here, however, they are associated with 

 shale partings and beds of impure limestone. 



In North-west Wyoming the outcrop of the Dakota Sand- 

 stones occupies a considerable area on the east and west flanks of 

 the Big Horn, Kattlesnake, and Owl Mountains. The beds 

 consist essentially of quartzose sandstone, with a few minor 

 shaly beds. The base is formed of a conglomerate, in which 

 the pebbles and matrix are more or less silicified. According to 

 the researches of the United States Geological Survey, the 

 Dakota Sandstone in Colorado is " a white quartzitic sandstone 

 " with a fine-grained conglomerate at the base, formed of very 

 *' well-rounded pebbles of the most dense and resisting siliceous 

 *' material, generally light or dark chert and jasper 



" The white quartzite generally occurs in one or two 

 ■" benches, with seams of clay near the middle. The conglome- 

 *' rate at the base of the granite is usually 2ft. to 5ft. thick." * 



In South Nebraska and a portion of Kansas, the Dakota 

 Sandstone is more or less altered into quartzite. The quartzite 

 type is fortunately exceptional, but such as it is it must play an 

 important part in preventing the absorption and transmission of 

 water along certain portions of the Dakota Sandstones. 



The next succeeding group is the Colorado, consisting 

 of black clayey shales, which reach a maximum thickness of 

 5,000ft. 



* Evidence of the Director of the United States Geological Survey, before the 

 Committee of the House of Representatives on Irrigation, Feb. 27, 1890 ; Eleventh 

 Annual Report, Geol. Surv., Pt. 2, Washington. By authority : 1891. 



