PETROGRAPHY 635 



dark grayish black. It is composed chiefly of greenish black pyroxene 

 and white to gray plagioclase feldspar, with scattered grains of magnetite. 

 In the coarser varieties the presence of orthoclase can generally be recog- 

 nized in the hand specimen — locally in large amount— and visible quartz 

 is commonly associated with it. 



While there is some variation in the proportions of the pyroxene and 

 feldspar, in many places they are approximately equal or one is not 

 greatly in excess of the other. As a rule, the elongated feldspars are in 

 contact with one another and the pyroxene in broader detached areas fills 

 the spaces between, constituting the typical diabasic texture. Less com- 

 monly the pyroxene is in such excess that the feldspars are more or less 

 isolated in it, producing an ophitic texture. Grains and crystals of mag- 

 netite are included in both of these constituents and minute needle-like 

 crystals of apatite occur as inclusions in all three. In nearly all speci- 

 mens of the normal granular diabase micrographic intergrowths of quartz 

 and orthoclase, with here and there also separate grains of these minerals, 

 occupy small scattered areas among the elongated plagioclase laths. Not 

 uncommonly a little biotite accompanies the pyroxene and magnetite, but 

 small grains of pyrite and chalcopyrite are rare. 



Feldspathic diabase, or anorthosite. — With the decrease of pyroxene, 

 plagioclase becomes the dominant constituent of the rock. Pyroxene con- 

 stitutes only about one-third and one-fourth of the rock, respectively, in 

 specimens from near the eastern border of the main sheet east of Gettys- 

 burg and from the cross-cutting body that extends westward from it at 

 the extreme southern border of the Fairfield quadrangle, G miles south- 

 west of Gettysburg. At the latter locality the pyroxene is reduced to 

 small grains and stringers among the relatively large feldspars, and the 

 rock from both jdaces is light gray to nearly white in color. Both contain 

 magnetite and minute needles of apatite, but a little micropegmatite and 

 a few scattered grains of quartz are found only in the latter. A remark- 

 able association of extreme types is shown in specimens from the thin 

 sheet 114 miles southeast of Bendersville, where a rock having the mega- 

 scopic characters of this feldspathic facies is closely accompanied by the 

 dense black basaltic type. A similar basalt from the same sheet three- 

 fourths of a mile farther east proves on examination with the microscope 

 to be an olivine basalt. 



Quartz diabase. — Micrographic quartz-orthoclase and plagioclase in ap- 

 proximately equal parts are the chief constituents of coarse-grained white, 

 pink, and gray facies of the diabase in many localities. Pyroxene forms 

 less than one-half and even less than one-fourth of the rock, with the 

 usual small amount of magnetite and apatite. Quartz and orthoclase are 

 XLVI — Bull. Geol. Soc. Am., Vol. 27, 1915 



