152 RE-EXAMINATION OF AMERICAN MINERALS. 



" The individual veins of this rather numerous group are remarkable for 

 their general mutual parallelism, their average course being about N. 31° — 

 35° E. by compass, and not at all coincident with that of the belt of country 

 which embraces them. They are true lodes or mineral injections, filling so 

 many dislocations or fissures transverse to the general direction of the strata 

 which they intersect. The metalliferous belt ranges not far from the boundary 

 which divides the gneissic or metamorphic rocks of Chester County from the 

 middle secondary red shale and sandstone strata. 



" This vein varies in thickness from a few inches to about two and a half 

 feet, and we may state its average width at not less than eighteen inches. It 

 is bounded by regular and well-defined, nearly parallel walls, the prevailing 

 material of which is a coarse, soft granite, composed chiefly of white feldspar 

 and quartz. 



" It would seem to be a pretty general fact that such of these veins as are 

 confined entirely or chiefly to the gneiss bear lead as their principal metal ; 

 whereas those which are included solely within the red shale are character- 

 ized by containing the ores of copper. But the zinc ores — viz., zinc-blende 

 and calamine — prevail in greater or less proportions in both sets of veins, 

 existing perhaps in a rather larger relative amount in the copper-bearing 

 lodes of the red shale. 



"The gneissic strata of the tract embracing this group of lead-bearing 

 veins seem to differ in no essential features from the rest of the formation 

 ranging eastward and westward through this belt of country. Here, as 

 elsewhere, they consist chiefly of soft, thinly-bedded, micaceous gneiss; a 

 more dense and ferruginous hornblendic gneiss; and thirdly, a thicker 

 bedded granitic gneiss, composed not unfrequently of little else than the 

 two minerals, quartz and feldspar. 



" Penetrating this quite diversified formation are innumerable injections 

 of various kinds of granite, greenstone-trap, and other genuine igneous rocks. 

 The granites, as throughout this region generally, consist for the most part 

 of a coarse binary mixture of quartz and opaque white feldspar, tending 

 easily to decomposition. This rock abounds in the form of dykes and veins, 

 sometimes cutting the strata of gneiss nearly vertically, but often partially 

 conforming with its planes of bedding for a limited space, and then branching 

 through or expiring in it in transverse or tortuous branches. A not uncom- 

 mon variety of granitic dyke is a simple syenite composed of quartz, greenish 

 semi-translucent feldspar, and a smaller proportion of dark-green hornblende. 

 A soft, white, and partially decomposed granite is a very frequent associate 

 of the stronger lead-bearing veins, particularly in their more productive 

 portions; but this material belongs, in all probability, not to the ancient 

 granitic injections of the gneiss, but to those much later metalliferous 

 intrusions which filled long parallel rents in that formation with the lead- 

 ores and their associated minerals. 



"The gneissic strata and their granitic injections throughout this district 

 display a softened, partially decomposed condition, extending in many places 

 to a depth of twenty fathoms. 



