GEOLOGICAL SOCIETY OF PENNSYLVANIA. 241 



rest upon the grauwacke series, which prevails to the 

 east of the Cumberland mountains ; one of the upper- 

 most of the above mentioned strata, is nearly wholly com- 

 posed of Strophomenes rugosa. These strata contain- 

 ing organic remains alternate with strata which contain 

 no fossils, and are on our high lands, covered with a 

 stratum of a species of alum slate, which, in a mineralogi- 

 cal point of view, resembles that variety in which, in 

 Westrogothia, the Agnostus pisiformis is found. I 

 have compared our slate with a specimen from the above 

 mentioned European locality, which is covered with the 

 A. pisiformis; and I found it similar. Some parts 

 eifervesce a little with acids ; other parts do not possess 

 that quality. This shale, which is mostly perfectly black, 

 straight, foliated, without lustre, passes sometimes into a 

 glossy slate with eurvated and distorted foliated struc- 

 ture. We never found any organic remains, vegetable 

 nor animal, in it; it lies in a conformable stratum upon the 

 limestone series already described. It is sometimes 

 bituminous, and contains even small seams of coal, which, 

 though bituminous, resembles in its external appearance 

 anthracite. It contains also nodules and particles of 

 pyrites. 



Upon the stratum of alum slate follows a stratum of 

 limestone, which often contains green earth ; it is mostly 

 of a sublamellar texture, in some places remarkably fetid, 

 and characterised by several genera of encrenites. I 

 found also different genera of trilobites, Spirifer caspida- 

 tus and large hamites. This stratum may be traced over 

 a large extent of ground, as in Davidson, Williamson, 

 Maury, Smith, White, Overton, and several other coun- 

 ties, lying always in the same geognostic position, that is 

 covering the alum slate. The fossils which it contains, 

 particularly the radiaria, are very numerous (sometimes 

 the whole stratum is made up of them), and are some- 

 times calcareous and often entirely changed into silex. 



1.—2 F 



