140 THE MINERAL RESOURCES OF PRINCE GEORGE'S COUNTY 



places the Potomac deposits contain molding sand of fair grade, but 

 it has not been used to any great extent. 



Locally the Lafayette and Pleistocene sands are rich in ferrugi- 

 nous matter, which in some places cements the grains together, form- 

 ing a ferruginous sandstone. Sands of this character possess a dis- 

 tinct value for road-making purposes, as they pack readily and make 

 a firm road bed. Where the material can be easily obtained in large 

 quantities, good roads can be very economically constructed with it 



THE GRAVELS. 



The Pleistocene, Lafayette, and Potomac formations contain 

 numerous beds of gravel widely distributed throughout the region. 

 Those of the Pleistocene and Lafayette deposits are generally rich 

 in iron, which acts as a cementing agent, thus rendering them of 

 considerable value as road metal. There are numerous gravel pits 

 in the eastern part of the District of Columbia in deposits belonging 

 to the Sunderland and Lafayette formations, and elsewhere in the 

 vicinity of Washington there are smaller pits in deposits of Wicom- 

 ico and Talbot age. 



THE BUILDING STONE. 



Prince George's County contains few beds of building stone of 

 much importance, yet in places materials occurring within the region 

 have been used locally. The granite-gneiss is the best building stone 

 of the region and furnishes good material for foundations and other 

 rough work. It is schistose and consequently can not be obtained in 

 large masses, but for that reason can be very easily quarried. Some 

 of the more massive beds furnish stones suitable for building, and in 

 places, where the beds are thinner and more micaceous, flagstones 

 can be obtained. 



Although the Coastal Plain formations of the region are composed 

 almost entirely of unconsolidated materials, yet locally indurated 

 beds are not uncommon. In the absence of any better stone these 

 indurated ledges furnish considerable material for the construction 

 of foundations and walls. The best stone of this class is the firmlv 



