S2 THE GEOLOGICAL STRUCTURE, ETC. 



The eastern edge of the basin is exposed in Eastern 

 Nebraska, and North-eastern Kansas, as well as in the south- 

 east corner of Dakota. Along a part of the eastern edge the 

 overlying impervious Colorado Shales overlap ; were it not for 

 this fact, since the western outcrop of the Dakota Group is over 

 3,000ft. above the eastern, the water would not accumulate, but 

 would escape at the lower eastern rim almost as fast as it could 

 be absorbed at the western. 



At Devil's Lake, 1,470ft. above sea level, a supply of 204,000' 

 gallons per day was obtained from a depth of 1,511ft., in the 

 upper part of the sandy series, which consist of a very fine 

 loose white sand, which was penetrated for 80ft. 



A series of calculations has been made by Mr. Warren 

 Upham, of the United States Geological Survey, from the 

 pressure recorded by the wells drawing their water from the 

 beds at the base of the Cretaceous, showing the height to which 

 the water would rise if confined in a tube. These calculations 

 show that in the States of North and South Dakota the water 

 would rise to heights varying from 35ft. to 432ft. above the mouth 

 of the bores. In every case the height of the head of water is 

 far less than the known altitude of the outcrop of the water- 

 bearing series ; the difference may be accounted for by the 

 frictional resistance of the rock through which the water flows. 



From Colorado northwards to the Canadian frontier the 

 outcrop of the Dakota Sandstone reaches the surface on the west 

 by a very steep curve, thus presenting only a narrow exposed 

 belt, which is estimated to occupy an average width of only 

 800ft. 



The Tertiary rocks of the Great Interior occupy the eastern 

 portion of New Mexico, Colorado, Wyoming, the country to the 

 south of the Cheyenne Siver in South Dakota, three-quarters of 

 Nebraska, and the western portion of Kansas, Oklahoma and 

 Indian Territory. 



The beds occur in the form of a tableland identical with 

 that of the Llano Estacado previously described ; the eastern 

 edge of this tableland is about 300ft. below the level of the 

 western. The rocks forming this tableland are open porous 

 cross-bedded sandstones, sands and gravels, covered in places by 

 a thin mantle of marl which is not entirely impervious to water. 



