322 WATSON AND CLINE IGNEOUS DIKES IN VIRGINIA 



ably phenocrysts of light-colored minerals, although some of them hear 

 a striking resemblance to amygdaloidal cavities. The microscopic evi- 

 dence is not conclusive, but apparently favors the former view, that these 

 areas are phenocrysts. The areas range in size up to 3 centimeters by 

 2 centimeters respectively. 



Plate 9, figure 2, is a view of a camptonite dike exposed along the 

 public highway one mile north of Mossy Creek. 



The general appearance of the hornblende and augite developed as 

 phenocrysts is very similar, but they can generally be distinguished by 

 the usual characteristic difference in form and the more or less altered 

 condition of the augite, which is often surrounded by a border of sec- 

 ondary granular pyrite. Augite is frequently partially replaced by calcite 

 from alteration, which can either be seen or detected by acid; also in 

 the larger crystals of augite small scales of chlorite can sometimes be 

 detected. A crystal removed from the matrix usually crumbles during 

 the process into granules of altered minerals. Hornblende, on the other 

 hand, appears fresh, and never shows in fresh specimens of the rock any 

 indication of alteration ; even on weathered surfaces of the rock it stands 

 out in relief with brilliant luster and perfect crystal form. 



The phenocrysts of hornblende, except the needle-like forms of the 

 ground-mass, are mostly idiomorphic in outline, elongated in the direc- 

 tion of the c axis, and exhibit the usual faces 110, 010, Oil, with some- 

 times 101 as small faces. The minute, needle-like forms in the ground- 

 mass show the faces of the prismatic zone, but rarely the domes. The 

 largest phenocryst of hornblende observed in the hand specimens was 2 

 centimeters in length and 5 millimeters in thickness. On removal from 

 the matrix the crystal was broken open and found to contain a core of 

 quartz about 1 centimeter in length. The center of this core was pure 

 quartz, but as the margin was approached minute prisms of hornblende 

 appeared, which became more abundant until the quartz entirely gave 

 way to the hornblende substance of the surrounding crystal. 



Microscopic character. — The essential minerals of the camptonite as 

 seen in thin-sections under the microscope are hornblende, augite, feld- 

 spar (orthoclase and plagioclase), and analcite. Apatite, biotite, and 

 magnetite occur in appreciable microscopic quantity, while titanite, rutile, 

 and quartz are developed sporadically. 



Hornblende is very abundant, developed in large, well-formed crystals 

 and in small microliths scattered through the rock-mass. It is highly 

 pleochroic; a; = light brown, ?/ = pale yellowish-brown, and z = deep 

 brown. The inclination of c to z is about 20° and the variety of am- 

 phibole is common basaltic hornblende, which is confirmed by the sub- 

 joined chemical analysis. 



