146 THE MIXEEAL EESOLKCES OF PEIX'CE GEOEGE's COU2s"TY 



their entire supply of water, which is usually of excellent character. 

 The spring water, as also that in wells, is in places highly charged 

 with mineral matter, particularly iron, sulphur, and salt, and some 

 such waters have been placed on the market. The most imjDortant 

 mineral springs of the county from which waters have been sold 

 are the Bladensburg Spa at Bladensburg and the Algonquin Springs 

 at Oxon. 



SHALLOW WELLS. 



N^early all the water supply of the county is derived from shallow 

 w^ells, varying in depth from 15 to 35 feet. The water is contained 

 in the rather coarse sand or gravel bed so commonly forming the 

 basal stratum of the Pleistocene and Lafayette deposits. So gen- 

 erally is this the case that the depth of the shallow wells is usually 

 a very good indication of the thickness of the surficial deposits. 

 The surface w^ter very readily penetrates the rather coarse surface 

 materials until it reaches the less permeable underlying sedimentary 

 or crystalline rocks. While some of it continues its downward course 

 into these harder rocks a great deal flows along on their upper sur- 

 face until it finds its way gradually into the streams. Hence wells 

 sunk to this level are practically assured of a supply of water which, 

 while seldom large in flow, is in seasons of average rainfall capable 

 of furnishing sufficient water for ordinary purposes. Such shallow 

 wells are necessarily dependent almost entirely on the amount of 

 water which percolates through the Columbia and Lafayette deposits 

 after rain storms, and are thus apt to be affected by droughts. After 

 periods of heavy rainfall the water may rise in the wells mthin a few 

 feet of the surface and then is very roily. At other times the wells 

 may become dry, yet this does not often occur because of the fairly 

 equable distribution of rainfall during the year. The supply is less 

 variable over -the broad divides or on level ground, where water is 

 always nearer the surface, than in the regions of narrow stream 

 divides, where the water finds an easy exit to the streams. In some 

 places on the narrow divides in proximity to the major streams, it 

 is necessary to sink wells to the depth of 100 feet or more in order 

 to obtain a permanent supply of water. 



