356 The American Geologist. December, i89u 
formly speckled with white flecks of feldspar. It is a medium 
grained diabase revealing apparently equal amounts of the 
(lark colored ferro-magnesian minerals and white feldspar, in 
which occurs a fair sprinkling of finely divided grains of 
pyrite. The light colored feldspar and quartz can be readily 
distinguished from the dark colored ferro-magnesian minerals. 
Microscopically, the rock is an aggregate of somewhat idio- 
morphic lath-shaped feldspars, with the interstices filled with 
allotriomorphic plates of augite, some olivine, magnetite and 
quartz. The olivine displays the usual alteration to serpen- 
tine along certain lines. The augite varies from colorless to 
the merest tinge of green in transmitted light, is generally non- 
pleochroic, and shows patches of uralite and chlorite as altera- 
tion products. A number of the best plates of augite showing 
OcPx) cleavage gave an average extinction, with this plane. 
of 33°-35°. The OcP cleavage was rather imperfectly devel- 
oped. The parting parallel to ocPoo was well developed, 
imparting, in most cases, somewhat of a diallagic appearance 
to the mineral. 
Analyses of the Augite. 
I 
SiO^ 50.71 
AUO3 3-55 
Fe^Oa 
FeO 1530 
MnO 0.81 
CaO 13-35 
MgO 13-63 
Na20 K ,0 
K.O )'-^^ 
Igni 1. 17 
II 
III 
49-33 
48.33 
9-15 
4.41 
0.27 
*IO.OI 
9-05 
16.36 
20.51 
14.58 
17. II 
0.55 
0.19 
0.25 
100.00 99.73 100.87 
I. Augite in West Rock, Conn., Hawes, G. W., Proc. U. 
S. Nat. Mus., 1881, p, 132. 
II. Augite in hypersthene diabase, Culpepper Co.. A'a., 
Campbell and Brown, Bull. G. S. A., 1891, 2, 344. 
III. Augite in olivine diabase. Chatham, Va., Watson. T. 
L., Am. Geol., 1898, 22, 88- 
A few scattered shreds of secondarv biotite were noted. 
