BY A. GIBB MAITLAND, C.E., F.G.S. 31 



To the Colorado succeed the Montana Beds, which are 

 made up of drab clay shales with, near the top, some arenaceous 

 strata, yielding overflowing water at the towns of Greely and 

 Eanton. The maximum thickness of the group is 8,000ft., but 

 it appears to thin out rapidly to the east. 



The uppermost group of the North American Cretaceous is 

 the Laramie, At the base of these beds is a very persistent 

 coarse sandstone or pure conglomerate, which yields water ; the 

 highest beds consist of clay shales and sandstones, with thin 

 coals. In Wyoming, the group attains a thickness of several 

 thousand feet ; in North-east Colorado it does not exceed 750ft. ; 

 in South-east Colorado and North-east Mexico, the thickness 

 has increased to 1,800ft. 



For our purpose it may be said that the North American 

 Cretaceous rocks consist of a porous sandy series at the base, 

 overlaid by a varying thickness of watertight clay shales. 



The structure of these beds in the region in question is 

 (omitting all minor details) that of almost one-half of a synclinal 

 trough, whose western rim is about 5,000ft. above the eastern. 

 The water supply of the Dakota Sandstone is received along 

 the continuous outcrop of the group in the foothills of the 

 Rocky Mountains. 



In the State of Dakota deep artesian wells occur along a 

 belt of country extending from Yankton to Devil's Lake, and 

 the greater part of the watershed of the James River. 



Wherever borings have penetrated the Dakota Sandstone, 

 artesian water has been found at depths varying from 900ft. to 

 1,500ft. In the south of Dakota the water-bearing strata are 

 arranged in a low broad synclinal trough, with its axis approxi- 

 mately north and south. 



The western edge of the catchment area in Dakota is along 

 the flanks of the Black Hills, to which the metamorphism of 

 the group does not extend. These hills which have an elevation 

 of from 5,000ft. to 8,000ft. above the sea level, send practically all 

 their drainage eastward, conveying the rainfall directly to the 

 outcrop of the water-bearing beds. These porous beds form an 

 escarpment round the hills, while the underlying impervious 

 Trias- Jura Strata have been so denuded as to form a huge ditch 

 along the inner edge of the sandstones which aids in conveying 

 to, and keeping the water in the Dakota Sands. 



