406 Mr. Weaver on Floetz Formations. [Dec. 



but they are often also rapidly inclined, or gently undulated, and, 

 generally speaking, in a position unconformable to the older 

 formations, upon which they repose ; appearing sometimes also 

 in the form of isolated caps or subconical hills. In other dis- 

 tricts, the relations are similar. Thus in the tract between the 

 forest of Thuringia and the Rhbn, M. Heim describes the shell 

 limestone as being there generally horizontal, to the top of 

 the highest hills ; but in some parts, the strata are variously 

 inflected, and elevated even in the vertical position. The mean 

 height of the shell limestone in the forest of Thuringia is esti- 

 mated at 1000 feet, but in the Geba mountain, it reaches to 1300 

 feet. 



Near the surface, the strata of shell limestone vary between 

 half an inch and four inches in thickness; butata greater depth, 

 they acquire a thickness of six or eight feet. They are frequently 

 in a disjointed state, being traversed by fissures, nearly vertical, 

 mostly open, and sometimes several feet in width. 



The shell limestone is upon the whole distinguished by its 

 homogeneous character, its grey, yellow, and white colours, and 

 by the abundant remains of organized bodies which it contains, 

 mostly assembled in families. With the exception of a few 

 layers of sandstone, hornstone, or flint, it is in a great measure 

 free from siliceous matter. 



Its most remarkable varieties arise from an intermixture of 

 silex, alumine, calcareous spar, and oxide of iron, or accordingly 

 as it contains, or is destitute of, petrifactions. Some of these 

 varieties pass into sandstone, and others into marl. 



In Mansfeld, and the adjoining parts of Thuringia and Anhalt, 

 the pure limestone is commonly yellowish-grey, isabella-yellow, 

 or yellowish-white; more rarely smoke or bluish-grey; fracture, 

 generally fine splintery, or even, and flat conchoida!, and dull, 

 and, more rarely, granularly foliated in part. It is seldom of a 

 dark bluish-grey colour, approaching to black, in which case it 

 forms a black stripe in the middle of a stratum, the colour becom- 

 ing lighter and fainter toward the exterior. But some strata are 

 almost of a hornstone character.* 



Among the principal varieties of the shell limestone may be 

 noticed the following : 



a. Mountain-green, or greenish-grey limestone, of a soft 

 tender, fine sandy consistence ; 



0. Ochre-yellow limestone, tender, feeling like sandstone ; 



<. Foliated granular yellow limestone ; 



d. Ochre-yellow, dull, fine sandy limestone, with local assem- 

 blages of calcareous spar ; 



e. Porous limestone, resembling tuffstone, compounded of the 



* The preceding description accords very well with the general characters of the 

 JEnglijh lias limestone. 



