410 GEOLOGY OF THE DENVER BASK*. 



beds B and C. The source of water-supply is at A, and the proposed well at F. 

 Let E be supposed to represent the surface of the ground (and. for convenience, also, 

 the surface of the common ground- water) in one of the two supposed cases, and D the 

 surface in the other. The arrow springing' from the surface E represents the upward 

 tendency of the water in the porous bed, owing to pressure from the fountain-head, 

 while the arrow depending from the line D represents the downward pressure of the 

 ground-water, whose surface is represented by D, and is, it will be observed, more than 

 equivalent to the upward tendency due to pressure from the fountain-head. A How at 

 F ci mid very safely be predicted if the surface were as represented by D, while it might 

 be doubtful whether one could be secured if the surface were as represented by E. 



My attention was first directed to this consideration by observing that where 

 the intermediate country was elevated and had a high water-level, wells flowed at 

 heights suprisingly near theoretical estimates, almost no deduction for obstruction 

 and leakage being necessary, whereas in those cases where the opposite was true 

 there was a very considerable falling short of theoretical estimates. 



THE CONDITIONS OF STRUCTURE IN THE DENVER BASIN. 



Form and extent of the basin. — The form of the Denver Artesian Basin is 

 somewhat irregular, but the many observations of dip indicate it to be a 

 shallow syncline, the axis of which passes in the vicinity of the Platte 

 River at Denver and is approximately parallel with the trend of the 

 mountains. The western rim of the basin is sharply upturned and clearly 

 defined; the eastern, northern, and southern rims are of gentle and irreg- 

 ular rise, portions of them, especially the eastern, remaining open-lipped. 



The extent of the basin involving the upper zone of water-bearing 

 strata, that is, the Denver and Arapahoe sands, is coincident with that of 

 the formations themselves, although the area of flowing wells is necessarily 

 considerably less. The exploited region, with few exceptions, is a belt 

 about 10 miles in width, lying along either side of the Platte, and 25 miles 

 in length, reaching from the vicinity of Littleton, 10 miles above Denver, 

 to Henderson Island, 15 miles below. The several transverse sections of 

 the held (structure sheet, PI. IV) show the general character and config- 

 uration of the basin and the gentle dips which the strata everywhere have 

 when beyond the influence of the sharp fold at the immediate base of the 

 mountains. 



Position of the strata favorable to absorption. TllC shallow depth of tllC basill 



brings the upper water-bearing strata within easy reach of the surface by 



