420 GEOLOGI OF THE DENTEK BASIN. 



The average weighl of water absorbed per cubic foot of the above 

 sandstone is 8 pounds, or a trifle less than 1 gallon per cubic foot. 



For the strata of the Arapahoe and Denver formations ho determina- 

 tions have been made. The lower measures of the Arapahoe, aboul 300 

 feet, are of very coarse material, often conglomeratic, very friable, and 

 porous, and consequently considerably more absorbent than the series al 

 the base of the Laramie. The exposure of their vertical or highly inclined 

 portion is one-fifth shorter than that of the Laramie, bul their entire north- 

 western edge is available at a low angle of dip. The absorbing power of 

 the sandy series constituting the upper water-bearing /.one of the Arapahoe 



formation probably falls somewhat lielow that of the basal sandstone of 

 the Laramie per cubic loot. Its material is generally a little liner, and it 

 is of somewhat looser texture, hut it carries some argillaceous constituents, 

 which must lower the ratio slightly. Its absorbing area is rather greater 

 than that of the lower sandstones of the formation, since it lies at a less 

 angle of dip and often extends well out on the prairie. 



The shalv members of the Arapahoe, while distinctly argillaceous, 

 have a considerable proportion of arenaceous material disseminated 

 throughout their mass, and in addition not infrequently carry porous, 

 lenticular bodies of sandstone at several horizons. Though their arena- 

 ceous character doubtless renders the shales pervious and sometimes avail- 

 able as water-bearing strata, and the hoilies of sandstone increase still 

 more their capacity for water, yet the ratio of absorption of the shalv 

 member as a whole must fall far below that of the sandstones of the 

 formation. 



The porosity and absorbing power of the Denver beds underlying the 

 prairie is considerably lielow that of the older strata, their constitution 



being much less arenaceous. The formation is, however, sufficiently 

 absorbent to play an important part in the artesian economy of the Denver 

 region, especially in the matter of ground water. 



POWEB or' transmission in Till-'. W vna: BEARING STRATA. 



Closely related in importance to the absorbent power of different 

 varieties of strata is the power o( transmission. Like the former, this 



