148 THE MINERAL RESOURCES OF PRINCE GEORGE "s COUNTY 



surface in the nortliwestern portion of the county undoubtedly occur. 

 This underlying consolidated rock mass is frequently spoken of as 

 "bed rock." In general the crystalline rocks are less permeable 

 than the overlying deposits and consequently check the downward 

 passage of the percolating soil water, which tends to flow along on 

 their surface or to collect in depressions. The surface of these old 

 rocks dips rather uniformly to the southeast at an average rate of 

 more than 100 feet to the mile. Along this crystalline floor much 

 water flows to lower levels, and it therefore marks a good water 

 horizon. Several artesian wells in the Coastal Plain derive an unfail- 

 ing supply of pure water from this level. In Washington and the 

 near vicinity water is obtained at this horizon in several wells, of 

 which those at St. Elizabeth's Asylum are the largest. Five of the 

 six artesian wells that supply the water system of Hyattsville prob- 

 ably obtain water at this horizon, which is reached at a depth of 250 

 feet. Though the water will overflow, the yield is increased by pump- 

 ing. These five wells, together with another less than half as deep, 

 are all pumped together and yield 130 gallons a minute. 



Throughout the greater portion of the county this crystalline floor 

 can never be very important as a water horizon because of its great 

 depth. It was not reached in a 1511-foot boring about 1 mile south 

 of Meadows, and it is probable that it lies as much as 2000 feet below 

 tide over a large portion of the county. 



Waters of the Lower Cretaceous Formations. — The Potomac 

 deposits contain many beds of coarse material that constitute good 

 water-bearing strata. Some of these sand and gravel beds lie be- 

 tween impervious deposits and thus furnish the requisite conditions 

 for flowing artesian wells. Within the District of Columbia the 

 beds belonging to the Potomac group are the principal water-bearing 

 formations. The water does not seem to come froro! any one horizon 

 of wide distribution, as is shown by the varying depths at which it is 

 reached and by the failure to obtain any water in these beds at cer- 

 tain places. Wells that were unsuccessful in finding a satisfactory 

 supply of water were the 360-foot well at the ice works and the 133- 

 foot well at the Mount Vernon apartment house. On the other hand, 

 at Hyattsville and in the vicinity there are several wells with small 



