138 THE MINERAL RESOURCES OF PRINCE GEORGE's COUNTY 



discussed under the headings Cretaceous, Eocene and Miocene, and 

 Lafayette and Pleistocene clays. 



Cretaceous Clays.- — The clays of the Potomac group are the most 

 valuable within the region under consideration. Each formation of 

 the group contains deposits of clay that are suitable for a variety of 

 uses. Some clays from the Patuxent have been employed for the 

 manufacture of common brick, fire brick, and terra cotta ; the Arun- 

 del contains clays adapted to the manufacture of common brick, 

 terra cotta, sewer pipes, and pottery; the Patapsco with its great 

 variety of clays furnishes material suitable for the manufacture of 

 common brick, fire brick, and other refractory ware, sewer pipes, 

 and pottery; and the somewhat less argillaceous Raritan formation 

 contains clays adapted to the manufacture of common brick, terra 

 cotta, and fire brick. 



Eocene and Miocene Clays. — Although argillaceous beds occur 

 very commonly in the Eocene and Miocene strata of the county, 

 they are generally too sandy to be of much economic importance. 

 Considerable lime, derived from the numerous fossil shells which are 

 either generally distributed throughout the sandy clay or concen- 

 trated in definite shell beds within the formations, also render these 

 clays of less value. They are, however, very accessible, being exposed 

 in the cliffs along the Patuxent River and in the valleys of tributary 

 streams, and if a way of utilizing them should be discovered, they 

 could be obtained in great quantities at little expense. The pink 

 clay at the base of the ^anjemoy formation, known as the Marlboro 

 clay, is the most valuable deposit of this group. It is about 25 feet 

 thick and is exposed at many places in the stream valleys between 

 Upper Marlboro and Piscataway. The clay is fairly plastic and 

 no doubt could be used for making pressed brick, but is not plastic 

 enough and is, besides, rather too sandy for pottery. 



Lafayette and Pleistocene Clays. — As already stated, the Lafay- 

 ette, Sunderland, Wicomico, and Talbot formations are generally 

 composed of coarse materials at the base of the deposits, with a rather 

 persistent loam cap which marks the last stage of deposition during 

 each particular submergence. This surficial loam, which is very 



