BY A. GIBB MAITLAXD, C.E., F.G.S. 33 



What may be called a modified ideal artesian water basin 

 occurs at Denver in Colorado ; the basin has a width of about 

 50 miles. Artesian water was obtained from the Arapahoe 

 conglomerate, the basal bed of the Miocene Tertiary, in 1883. 



This conglomerate, or gritty sandstone, varies in thickness 

 from 600ft. to 1,200ft. It is overlaid by a varying thickness of 

 sandy shales, clays, sandstones, and basalt. 



The Arapahoe beds reach the surface on the west at the 

 town of Golden, at an elevation of about 6,500ft., by an upward 

 curve, and thus present but a narrow outcrop. 



" The flow yielded by the first well was so large, and the 

 " water was of such superior quality for domestic use, that other 

 " wells were put down with great rapidity. There are now 

 " (1889) in the city and its vicinity about 300 wells. Many 

 " of the first wells had sufficient pressure to force the water into 

 " tanks, or to the tops of the highest buildings in the city, bu 

 " as the number of wells was increased the pressure and flow of 

 " the older wells began to diminish, and finally in the region 

 " where they are most closely grouped they have failed to furnish 

 " water without the aid of pumps. Outside the region of closest 

 " grouping pressure and flow have been diminished, but not to 

 " so great an extent. Deep wells are stiU bored at Denver, but 

 "not with the expectation of obtaining an artesian flow."* 

 (Plate I., fig. 2). 



The amount of water which can be taken from an under- 

 ground reservoir of this nature depends not upon the total 

 quantity stored therein, but the rate at which it can flow 

 through the rocks, and after the first wells have drawn upon 

 the accumulated supply the amount which can be taken after- 

 wards is governed by the rate at which the water can travel to 

 the well. 



In the Atlantic and Gulf Border regions there is only one 

 side of the synclinal trough present. 



The Great Basin of the Interior is really in the form of one- 

 half of a synclinal, whose western rim is 3,000ft. above the 

 eastern. The Tertiary rocks, which dip eastward at about 10ft. 

 per mile, absorb water like sponges, and their position upon a 



■^ Evidence of the Director of the United States Geological Survey before the 

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

 Ann. Rep., U.S. Geol. Surv., pt. 2, p. 262. Washington. By authority : 1891. 



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