636 STOSE AND LEWIS TRIASSIC IGNEOUS ROCKS 



commonly present also in separate grains. Numerous large grains of 

 quartz are distinctly visible in several places, and in some specimens crys- 

 tal terminations are seen in miarolitic cavities. Scattered grains of yel- 

 lowish brown titanite are readily recognized in the coarse-grained white 

 variety from the Biggs quarry, 1 mile south of Gettysburg. In this rock, 

 also, the pyroxene is in the form of lath-shaped prisms rip to 20 milli- 

 meters in length and a little biotite is visible. Decrease in plagioclase 

 results in the transition to micropegmatite, and in some localities aplite 

 is also associated with this fades. 



Micropegmatite. — Consisting chiefly of quartz and orthoclase in micro- 

 graphic intergrowth, this variety of pink to pinkish gray rock is much 

 like the quartz diabase described above and shows gradations into it. The 

 texture ranges from medium fine to coarse grained. Quartz and ortho- 

 clase in large separate grains are common constituents, while plagioclase 

 is very subordinate and in some the pyroxene is almost entirely lacking. 

 This type is well represented at several localities. 



Aplite. — -Associated with quartz diabase and micropegmatite is an ap- 

 litic facies of the rock consisting of a fine-grained aggregate of quartz and 

 orthoclase, with subordinate plagioclase, shreds of pyroxene, and scattered 

 grains of magnetite and apatite. Numerous irregular grains of titanite 

 occur in the pink aplite, which is abundant in the cross-cutting body 6 

 miles northeast of Gettysburg. 



Hypersthene diabase. — Both orthorhombic and monoclinic pyroxenes 

 are present in hypersthene diabase. Several specimens from the lower 

 and middle portions of the Gettysburg sheet and one from the apophysis 

 (3 miles to the northeast contain hypersthene in varying proportions, 

 from about one-fifth to more than one-half the total pyroxenes, and in all 

 cases these minerals exceed the feldspars in amount. A little biotite is 

 found also in all of them, but the micrographic intergrowth of quartz and 

 orthoclase is a constituent of only one specimen. The hypersthene shows 

 the usual pleochroism in shades of red and green. A specimen of olivine 

 diabase from near the bottom of the Gettysburg sheet also contains a little 

 of this mineral. 



Olivine diabase. — Fresh rounded grains of olivine of all sizes, from 

 minute granules to about the average texture of the rock, constitute about 

 10 per cent of the bulk of the olivine diabase above referred to from the 

 Gettysburg sheet. The rock is of medium fine texture and of dark brown- 

 ish black color. Hypersthene is present and a small amount of biotite, 

 but micropegmatite and quartz are absent. Otherwise the rock is similar 

 in all respects to normal diabase. 



