PETROGRAPHY 639 



Magnetite. — Magnetite commonly forms 5 per cent of the rock or less. 

 The grains and crystals are generally smaller than the associated pyrox- 

 enes and feldspars, and some of them form inclusions in both of these 

 constituents. Lattice-like and dendritic skeletons are common. Some of 

 these are beautiful aggregates of minute octahedrons. Secondary magne- 

 tite is commonly abundant among the alteration products of pyroxene. 

 Masses that are molded irregularly about and among the plagioclase crys- 

 tals are doubtless of this character — at least in part. 



Biotite. — Small amounts of biotite are scattered through the finer 

 grained contact facies and larger irregular flakes are clustered about the 

 magnetite and pyroxene of the coarser grained rock. In places it is ap- 

 parently secondary after pyroxene. It is altered in part to chlorite. The 

 great bulk of the normal diabase contains little or no biotite. 



Olivine. — Olivine occurs in the finer grained rock chiefly as scattered 

 phenoerysts, which are generally altered, in part at least, to yellow or 

 yellowish brown serpentine. Somewhat rounded and irregular grains are 

 common, but some have crystal outlines. 



Apatite. — Apatite in minute cr} r stals occurs in all varieties of the dia- 

 base and forms inclusions in all the other constituents, but chiefly in the 

 feldspars and quartz. It forms slender hexagonal prisms, with maximum 

 dimensions of 0.07 by 2.0 millimeters, although most of them are much 

 smaller and appear needle-like under the microscope. 



Pi/rite and chalcopyrite. — Small grains of pyrite and chalcopyrite are 

 occasionally seen in hand specimens of the diabase, but they are rarely 

 recognized in the thin sections. 



Titanite. — Grains and crystals of titanite are abundant in the pink 

 aplite facies of the apophysis 6 miles northeast of Gettysburg and in the 

 coarse white quartz diabase south of Gettysburg. The latter rock contains 

 elongated jjrisms of pyroxene and radial clusters of epidote in crystals up 

 to 1 5 millimeters long. 



Tachylite, basalt glass. — Dark-brown glass forms an abundant ground- 

 mass in the black olivine basalt southeast of Bendersville. It is thickly 

 sprinkled with granules and crystals of magnetite and in places is black 

 and opaque with them. The olivine basalt east of Chestnut Hill has a 

 vesicular crust 8 millimeters thick, which is probably glass in part; but 

 the dense rock immediately adjacent to this crust is entirely crystalline. 



CHEMICAL COMPOSITION 



Composition of various types of diabase. — Analyses of Triassic dia- 

 base from various parts of the Atlantic slope agree in general character, 

 but show considerable variation corresponding to their diverse mineral 



