JSIAKYLAXD GEOLOGICAL SURVEY 141 



cemented white sandstone occurring in the liaritan formation about 

 1 mile north of Collington. The shell beds of the Aquia in the 

 vicinity of Upper Marlboro are so firmly consolidated that they 

 furnish building stone, which though of poor grade is nevertheless 

 suitable for rough work. The gravel bands of the Lafayette and 

 Pleistocene are, in many places, so firmly cemented by iron oxide 

 as to form pebble conglomerates of considerable strength. 



THE MARLS. 



Glauconite Marls. — The Eocene and Upper Cretaceous formations 

 of the county are rich in deposits of glauconitic marls, which are of 

 value as fertilizers. From New Jersey to North Carolina such 

 deposits have been worked spasmodically since the early part of the 

 last century, when their value was first determined, yet their impor- 

 tance in enriching the soil has never been generally recognized. They 

 consist of quartz sand with an admixture of many grains of glau- 

 conite, a soft green mineral which is essentially a hydrous silicate 

 of iron and potassium. On account of the glauconite, the marls are 

 green in color and are commonly known as ''greensand marls." They 

 are rich in calcium carbonate derived from the shells which are 

 abundant in the deposits, and chemical analyses usually show the 

 presence of small amounts of mineral phosphates. The marls thus 

 contain three important plant foods — potash, lime, and phosphates. 

 Altogether these constitute only a small percentage of the entire 

 content of the deposits, yet wherever the marls can be obtained at 

 low cost, they furnish economical means for increasing soil fertility. 

 Where the glauconite marls have been used it is claimed that their 

 beneficial effects is much more lasting than that obtained by means 

 of artificial fertilizers. Within the county many Eocene and Upper 

 Cretaceous beds rich in glauconite outcrop along the sides of the 

 stream valleys, extending in a belt diagonally across the county from 

 the Patuxent River to Mattawoman Creek. 



Shell Marls. — The shell marls of the Miocene and Eocene forma- 

 tions also possess valuable fertilizing properties for soils deficient in 

 lime. In some places the shells are mixed with so much sand that 



