632 STOSE AND LEWIS — TRIASSIC IGNEOUS ROCKS 



dense black faeies, in places vesicular and having a glassy ground-mass; 

 (9) olivine basalt, the dense black variety, with abundant olivine. 



The areal distribution and delimitation of these varieties can not be 

 determined without much more detailed work in the field and the aid of 

 great numbers of thin sections — work that is out of the question, even if 

 desirable, at the present time. Furthermore, the boundaries, in part at 

 least, would have to be drawn arbitrarily, since gradational faeies exist in 

 some places, as shown by thin sections already in hand. 



Strongly contrasted faeies are apparently closely associated in places, 

 as in the occurrence of highly . f eldspathic along with olivine-bearing 

 basaltic types southeast of Bendersville. In the larger bodies the proba- 

 bility of some gravitational differentiation seems to be indicated, similar 

 to that observed in the Palisades of the Hudson. This is particularly true 

 of the Gettysburg sheet, where acid felclspathic and quartzose faeies seem 

 to dominate in the middle and upper (westerly) parts, although not to 

 the exclusion of some more basic rock, while black, heavy hypersthenic and 

 olivinic faeies are typically developed near the bottom along the easterly 

 contact. The outcrops here, however, arc neither so continuous nor so 

 fresh as in the Palisades, and the extent to which such differentiation has 

 occurred would be difficult to determine. 



GENERAL CHARACTER. OF THE DIABASE 



The larger intrusive masses, both in the sheets and the cross-cutting 

 bodies, present a different appearance in the main from the dikes and 

 thin sheets, although they are obviously parts of the same original magma. 

 Indeed, some of the most strongly contrasted types are parts of one con- 

 tinuous mass, and all of them are doubtless united in a similar manner at 

 no great depth. As a rule, the larger bodies of diabase are coarse grained, 

 with colors ranging from light pink and gray to dark gray, and some of it 

 is quarried and marketed as granite. The dikes and thin sheets, on the 

 other hand, are fine-grained to dense rock and are nearly black. 



In normal diabase, which is the prevalent type, the most important 

 constituents arc greenish black pyroxene and a white or grayish plagio- 

 clase. The pyroxene commonly preponderates, but" in many places the 

 two minerals are approximately equal and locally the feldspar is in excess. 

 Crystals and irregular grains of magnetite and minute slender needles of 

 apatite are rather plentiful and there is generally a little quartz and 

 orthoclase. The darker varieties of the rock contain locally much hy- 

 persthene or olivine or both, while in the lighter faeies quartz and ortho- 

 clase abound — as a rule, intergrown in greater part as micropegmatite. 

 Here and there biotite is present in much smaller amount and, far less 

 comraonlv, titan ite. 



