228 WATSON AND CLINE ROCKS OF THE BLUE RIDGE REGION 



indicate that it is near common hornblende in composition; the angle 

 c A 2. is about 23 degrees; absorption is z = deep brown, x and y light 

 brown. It shows no alteration. 



ILMENITE-APATITE GABBRO 



This type was not found in place, but is represented by abundant large 

 and small boulders littering the surface on the southeast slope of Rocky 

 Row Mountain between James River and the headwaters of Rocky Row 

 Run, in Amherst County. It is not possible, therefore, to say what its 

 field relations are to the other rocks. The rock is very dark in color be- 

 cause of the predominance of ilmenite and pyroxene, although feldspar 

 irregularly distributed through the rock is recognized, usually grouped 

 in irregular-shaped areas of about 1 inch in diameter. Apatite is devel- 

 oped in small crystals, many of which show hexagonal outline; when 

 crystal form is lacking the mineral is distinguished with difficulty from 

 the small grains of secondary epidote. Some secondary quartz, red garnet, 

 and chlorite can be distinguished in hand specimens. Indistinct foliation 

 is developed in the rock. 



Microscopic study of thin sections of the rock show it to be much 

 altered, the most abundant minerals of which are secondary, including 

 chlorite, amphibole, epidote, and quartz. However, in some thin-sections 

 the original minerals are sufficiently fresh to determine the composition 

 of the rock to be chiefly hypersthene, plagioclase (labradorite), some 

 orthoclase, ilmenite, apatite, diallage, and quartz. 



Hypersthene is abundant, but is much altered, the alteration being- 

 identical with that of the hypersthene in the syenite, with the exception 

 that considerable chlorite is developed in the alteration of the gabbro. 

 Diallage is less abundant than hypersthene and does not occur in all thin- 

 sections ; but when present it is fairly fresh, with only slight alteration 

 to probably amphibole along the cleavage and fractures. Plagioclase of 

 the variety labradorite is greatly altered, and in most cases little or none 

 of the original mineral remains. Some orthoclase is also present. 



The richness of the rock in apatite and ilmenite is a noteworthy feature, 

 which removes it from the ordinary gabbro, and in this respect the rock 

 resembles the gabbro-nelsonite (roselan close) of the Amherst-"N~elson 

 counties area, 42 an analysis of which is given on page 229. The apatite 

 is in large amount in all thin sections studied, developed both as separate 

 large rounded hypidiomorphic crystals and as small crystals inclosed in 

 the other minerals. It varies in quantity in different thin-sections and 



42 Watson and Taber : Bull. III-A, Virginia Geol. Survey, 1913, pp. 138-142. 



