342 H. D. CAMPBELL AND W. G. BROWN — COMPOSITION OF TRAP. 



railroad in Culpeper county, Virginia, about two miles north of Rapidan 

 station. This rock weathers into globular masses and contains olivine through- 

 out, so far as we could judge from making numerous thin sections. 



Description of the Hypersthene-Diabase. — The hypersthene-diabase has a 

 medium grain and is of a dark-gray color. The darker varieties have a 

 somewhat greasy luster. The unaided eye can detect two dark minerals, 

 the one nearly black, and the other deep honey-yellow or brown in a very 

 light-colored background. Sometimes the light material occurs as small 

 irregular veins running through the darker rock. 



In thin sections we were able to distinguish triclinic feldspar, diallagic 

 augite, hypersthene, biotite, apatite and occasional quartz, hornblende and 

 probably zircon. Black opaque grains were also present, and as these were 

 magnetic and some of them showed a trace of titanium, they were considered 

 to be magnetite and ilmenite. 



The structure is generally ophitic. It seems to be intermediate between 

 the granular structure of gabbros or norites and the porphyritic structure 

 of the holocrystalline varieties of the augite-porphyrites, shading sometimes 

 into the former, sometimes into the latter. It is owing to the predominant 

 ophitic structure alone that we place these rocks among the diabases. The 

 mineral composition would give them a place among the gabbros, for the 

 monoclinic pyroxene is diallagic. The hypersthene, however, more closely 

 resembles that found in the hyperstheue-andesites. These rocks afford an- 

 other illustration of the view that the difference between gabbros and dia- 

 bases is structural rather than mineralogical. 



Constituents of the Hypersthene-Diabase. — The feldspar makes up all of the 

 light material which is visible to the naked eye. In thin sections under the 

 microscope it appears principally as lath-shaped crystals, polysynthetically 

 twinned. In typical specimens these crystals are rarely larger than 1.5 mm. 

 in length by 0.3 mm. in width, the majority being much smaller than these. 

 Tabular crystals also occur, but they are not very common. Truly idio- 

 morphic crystals are rare. Zonal structure is frequently visible with crossed 

 Nicols, and the angle of extinction of the central part is greater than that of 

 the margins. Minute crystals of apatite and other inclosures occur here and 

 there. The angle of extinction, measured between twins which extinguish 

 equally on two sides of the twinning plane, was as large as 36° in a number 

 of instances, which points to the presence of anorthite. 



Two analyses of pure white material, separated by means of the Klein 

 solution, between the densities 2.672 and 2.704 gave the following results : 



