410 NATURAL HISTORY. 



geologists as a group of sandy marl, and remarkable for 

 numerous fossil vegetable remains, mostly those of the 

 Equisetum (Horse-tail) and Reed families. A kind of 

 carboniferous clay or limestone is occasionally met with 

 in this formation, but is little valued. 



Variegated Marl, with Gypsum frequently found in- 

 stratified -with the upper layers, is composed of indurated 

 clays of various colors. Gypsum is found in separate 

 strata ; is mostly red. but sometimes, although rarely, 

 white as alabaster. 



Letlen-koklej Germ., Clay Slate Coal. Lying under 

 the lowest strata of the Keuper Sandstone, and easily 

 recognized by its light gray color, this species of coal is 

 found in company with marl, gypsum, and alum-slate. 

 It is impure, clayey, fissile, and, in coming in contact 

 with the air, splits off into laminae ; occurs only in schis- 

 tose strata, and can not be used for burning. 



Muschelkalk Conchylian Limestone (shell-chalk) 

 lies under the Lettenkohle marls in strong layers, which 

 represent huge, compact masses of limestone ; is of a 

 dull gray or dark greenish-gray color, and commonly 

 contains, in great abundance, the remains of shells and 

 fragments of radiated animals and fishes. As it admits 

 of a fine polish, it is often used for table-slabs, etc. 

 Dolomite, or carbonate of magnesia, is often found super- 

 incumbent above muschelkalk ; and mineral springs 

 abound in this formation, as well as immense strata of 

 rock salt, which occur directly under the beds of shell 

 chalk. 



Rock Salt is mostly associated with gypsum, anchy- 

 drytes, or sulphate of lime (clay, sandstone, and calcar- 

 eous spar) ; in Europe, however, it usually occurs in the 

 new red sandstone, or associated with red marl, but is not 



