in Pennsylvania and Maryland. 491 



These rocks have been more generally sheared into slates 

 than the acid lavas. The chemical alteration which has gone 

 on in them is also in general greater. Still large masses of the 

 basic rocks have been but little altered and remain quite 

 massive. These, which are locally known as " copper rock," 

 are the only members of the volcanic series whose igneous 

 origin has been heretofore conceded. They are for the most 

 part very fine grained, vesicular flows, whose original structure 

 is still so well preserved that they may with propriety be 

 called basalts. 



The following analysis, also by Mr. C. H. Henderson, of a 

 massive greenstone from the Bechtel copper shaft, Russel 

 mine, is published by Dr. Frazer.* This is a normal basalt 

 analysis, indicating as little chemical change in the basic rocks 

 as the one given above does in the acid rocks. 



SiO„ 41-280 



A1 2 3 18-480 



Fe 2 3 . 9-440 



FeO 8-200 



CaO ' 7-040 



MgO 7-486 



Na„0 3-523 



K 2 2-208 



Ign. 2-740 



Total 100-397 



Basic volcanic rocks never exhibit so great a variety of struc- 

 ture forms' as characterize the more acid rhyolites. The South 

 Mountain basalts are usually homogeneous, dark to pale green 

 masses which rarely show any microscopical phenocrysts and 

 whose most constant feature is amygdaloidal structure. These 

 cavities vary greatly in size, shape, and abundance. They are 

 often elongated by flow-motion in the lava and are now filled 

 with a number of secondary minerals, the most abundant of 

 which are epidote, chlorite, quartz, and zeolites. Traces of 

 original glass or spherulitic structure (variolite) have not yet 

 been detected in the basalts. 



The mineral constituents of basic rocks are more subject to 

 alteration than those of acid ones. It could not be expected 

 that basalts so ancient and so vesicular as those of South 

 Mountain would escape all change, but it is a surprise in many 

 cases that this change has been so small. The ferro-magnesian 

 constituents have always altered to epidote, chlorite or serpen- 

 tine, but the structure is frequently preserved in its minutest 



* "Hypothesis of the structure of the Copper belt of South Mountain," Trans. 

 Am. Inst. Min. Engineers, vol. xii, p. 82, 1883-4. 



