On the Geology fyc. near West Chester, Pa. 1 5 



Art. II. — On the Geology and Mineralogy of the country 

 near West Chester, Penn. ; by J. Finch, M. C. C. &c. 



Chester County, in Pennsylvania, possesses much inter- 

 est to the geologist and mineralogist, on account of the 

 beauty, and variety of the specimens it affords. 



If a line be drawn from the battle ground near Chad's ford, 

 passing through West Chester in a northerly direction, and 

 extending beyond the bounds of the county, the following 

 formations may be noticed on the route. 



1 Gneiss, containing subordinate strata of hornblende 

 slate, serpentine, and limestone. - 8 miles 



2 Mica slate, - - - 2 do 



3 Primitive limestone, - - - 1| " 



4 Transition quartz rock, - - - H u 



5 Gneiss, and hornblende slate, - - 6 " 



6 Second or variegated sandstone, - - 4 " 



7 Newest flcetz trap, - - 1 " 



1. Gneiss. This is composed of the usual ingredients, 

 quartz, feldspar, and mica, arranged in slaty laminse, at an angle 

 of 70° to 80° to the horizon. On some of the hills, hard 

 masses of the rock may be seen, but, more frequently, it has 

 undergone decomposition to a depth of twenty or forty feet. 

 In the defiles formed by many of the roads, this'may easily 

 be seen, and the superior fertility of Chester county, is to be 

 attributed to this cause. The surface of the rock is distin- 

 guished by its undulating character, which forms a pleasing- 

 feature in the landscape. 



The hornblende slate, contained, in the gneiss formation, 

 is composed of quartz, feldspar and hornblende, the latter 

 mineral predominates and forms two thirds of the mass. 

 Frequently only feldspar and hornblende occur. It is slaty, 

 and the rock will usually break, though with difficulty, in 

 the direction of the laminse. Where it is abundant on the 

 surface of the ground, it is very troublesome to the farmers, 

 as it resists decomposition. The limestone occurs in several 

 strata, varying in breadth from thirty to one hundred feet, 

 and in some situations probably more. It is rendered la- 

 mellar by mica, and the laminae vary from two to twelve in- 

 ches in thickness. It is crystalline and contains magnesia. 

 Color, white, greyish white, reddish and blue. The strata of 

 limestone are probably contiguous with the gneiss, although 



