16 On the Geology §c. near West Chester, Pa. 



quarries have been opened in few places. In the interven- 

 ing spaces, it is in general too much covered with earth to 

 be worked to advantage. 



Serpentine. This occurs in masses at irregular distances, 

 and appears to form superincumbent masses reposing on the 

 gneiss. It is universally distinguished by its sterility, and the 

 fields, where it occurs have received the emphatic title of the 

 " Barrens. 1 ' There is a great difference in the external char- 

 acter of this rock, color usually blackish or yellowish green, 

 sometimes red. On the exterior it decomposes white, and 

 more rarely black. It is sometimes of a slaty structure and 

 distinctly stratified. 



2. Mica Slate. This rock passes into clay slate, covered 

 with a glaze of mica. The decomposition of the rock, yield- 

 ing mica and quartz, produces a porous soil, distinguished 

 by its sterility. By adding lime, it has been ameliorated. 

 The water on this tract, is remarkably pure and limpid. 



3. Primitive Limestone. This varies much at different 

 quarries. Color white, black and veined. Some varieties 

 contain much carbon. In strata, inclined 70° to 80° and fre- 

 quently contains fissures of unknown extent. It forms a dis- 

 trict of country called the great Valley, which extends from 

 the Schuylkill, to the Susquehanna. Soil fertile. 



4. Transition Quartz rock. This constitutes a range of 

 hills, forming the north west boundary to the great valley. 

 The rock is chiefly quartz, occasionally intermixed with feld- 

 spar, and colored of a slight red tinge, from oxide of iron. 

 Slaty, surface barren. Strata highly inclined. 



5. Gneiss, with hornblende slate, similar to that already 

 described. The yellow springs are situated on this forma- 

 tion, and from this source, derive their supply of chalybeate 

 water. 



6. Second, or variegated Sandstone. It presents the usu- 

 al varieties. The predominant rock, is an argillaceous sand- 

 stone, containing much oxide of iron ; it is easily quarried, 

 and makes a good building stone ; it alternates with slaty 

 sandstone, which decomposes on exposure to the atmos- 

 phere, and produces a soil easily cultivated. Inclination of 

 the strata, 1 0° to 20.° The lead mines of Perkiomen, are situ- 

 ated in this stratum, as are also, those in the vicinity of the farm 

 of Wm. Everhart, Esq. six miles south west of Unionville. 



7. Newest floetz trap ; sometimes resembles the primitive 

 hornblende slate, but is usually in amorphous masses, and 

 the hornblende is not so distinctly crystallized. 



