408 GEOLOGY OF TIIE DENVER BASIN. 



degree of porosity than to a permeable layer confined between strictly 

 impermeable layers. 



Such a distribution of materials as the above explains both the fre- 

 quency and the certainty with which flows are encountered in descending 

 through the formation, and also the peculiarity that in two wells tapping 

 the same horizon there may be a marked difference in yield or freedom of 

 How, or even entire absence of a flow in one of them. 



The Denver formation. This ClltCl'S lillt sliglltlv UltO till' artesian System 



of the field, on account of its shallow depth. The component beds are 

 conglomeratic, sandy, or argillaceous. Conglomerates and sandstones are 

 well developed over extended areas, but their appearance beneath the 

 prairies is more or less irregular and uncertain on account of proximity to 

 the base of the formation and the unconformability existing between this 

 and the underlying Arapahoe and Laramie formations. The (days of the 

 Denver series are usually arenaceous. The character and texture of all 

 the sediments vary from place to place and the yield of water changes 

 accordingly. 



AN UPPER CONFINING STRATUM. 



The porosity of the Denver and Arapahoe formations, not only in 

 their more sandy members but also in their clayey beds, considered in 

 reference to their position immediately beneath the surface and the gentle 

 slope of their strata, illustrates one of the primary elements of what Prof. 

 T. C. Chamberlin calls the confining stratum above. 1 His statement in 

 regard to this feature is as follows: 



The element to be recognized here is, 1 believe, essentially new to discussions of 

 the subject, viz, the height of the surface of the common groundwater iu the region 

 between the proposed well and the fountain-head. It is a familiar fact that the 

 common underground water stands at varying heights. Our common wells testify to 

 this. The subterranean water-surface is almost invariably higher than the adjacent 

 streams, and slowly works its way into them by springs, seeps, and invisible perco- 

 lation. Speaking generally, the underground water-surface rises and falls with the 

 rise ami tall of the land surface, only less in amouut. Now, if the subterranean water 

 in the region between the proposed well and its source — which we may call the cover- 

 area — stands as high as the fountain-head .except at the well, where, of course, it must 



'Filth Ami. Kept. L 7 . S. Geol. Survey, )>. 139. 



