10 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 66, 



except in the extreme northern part, where it coarsens perceptibly 

 to an average grain diameter of 2 mm. It is noticeable that where 

 the grain of the rock is coarse there are no pegmatites. 



Under the microscope the essential constituents of the diabase are 

 seen to be pyroxene and plagioclase. The texture is dioritic, both 

 the feldspar and the pyroxene tending to euhedral development. 

 Distinct dibase or ophitic textures are lacking, and, although the 

 proxene is not sufficient in amount to form a mesh, it has not crys- 

 tallized definitely after the feldspar, the crystallization of the two 

 minerals probably being more or less simultaneous. The texture is 

 illustrated in the photomicrograph, plate 4, upper. 



The feldspar, which is the most abundant constituent, occurs in 

 lath-shaped crystals variously oriented, although at one place diabase 

 was seen which had a parallel orientation of the feldspar yielding a 

 faint schistosity and tendency to cleavage in one direction. The 

 plagioclase is, for the most part, unaltered in the body of the rock 

 away from seams and veins, and is all twinned on the albite law with 

 medium broad twin lamellae. Grains showing also carlsbad and 

 pericline twinning occur but rarely. Chance sections suitable for 

 determination of the plagioclase by measurement of extinction angles 

 are rare. One section showing the combination of albite and carls- 

 bad twinning gave extinction indicating the composition AbgjAnes. 

 Study of the powdered rock by the immersion method shows the feld- 

 spar to be rather constant in composition, the optical properties 

 determined being : Biaxial positive ( + ) , 2 V large, indices of ref rac- 

 tion «= 1.555, j8= 1.560, 7=1.565, the composition indicated being 

 that of an acid labradorite, Ab^^Angg. Another determination on the 

 feldspar separated from the coarse phase at the north end of the 

 quarry gave |8= 1.563, corresponding to the composition Ab42An58. 



The alteration of the feldspar most frequently seen is by the 

 development of nests of rather coarse flakes of a micaceous mineral 

 of high birefringence as further discussed under the various types 

 of hydrothermal alteration below. 



The pyroxene is all characterized by a more or less distinct pinkish 

 brown color in thin section, probably caused by the titanium content. 

 That in the rock from the quarry is not pleochroic, although a part 

 of the pyroxene of the rock from the side of the Leesburg Pike shows 

 a faint pleochroism in pale violet brown and pale green. Normally 

 the pyroxene is transparent and free from inclusions. In many sec- 

 tions, however, much of the pyroxene is darker in color due to the 

 presence of numerous microscopic opaque inclusions distributed in 

 planes parallel to the basal pinacoid c (001) and probably connected 

 with incipient alternation. In sections normal to the prismatic elon- 

 gation the augite shows euhedral bounding by the unit prism m (110) 

 and the pinacoids a (100) and h (010) often with twinning parallel to 



