AREA OF NEPHELINE-BEARING ROCKS 323 
The first of these rocks belongs to the class of the augite- 
syenites, but is of a peculiar type. The augite is represented by 
two varieties which pass into one another. One is a purplish- 
brown augite, which frequently constitutes the inner portion of 
large individuals, and shades away into an outer border of green 
augite of the second variety. This green augite also occurs in 
separate individuals. Both varieties have high extinction angles, 
and the green variety is probably an aegerine-augite. In addi- 
tion to the augite, a small amount of deep bluish-green and 
highly pleochroic hornblende is present. The single sec- 
tion of this rock also contains a considerable amount of a 
mineral which has the high index of refraction and high double 
refraction of olivine, and which is destroyed by acid with 
gelatinization. 
The feldspars, which with the augites make up most of the 
rock, consist in part of orthoclase and in part of microperthite, 
and possibly anorthoclase, and usually possess a zonal structure, 
an outer border or rim of microperthite often surrounding an 
individual of orthoclase nearly free from intergrowths. Small 
quantities of pyrite and magnetite are also present, as also of a 
deep brown, almost opaque, non-metallic mineral, which is unat- 
tacked, even by prolonged treatment, with concentrated hydro- 
chloric acid, and which is probably one of the rarer rock-making 
minerals. 
The structure of the rock is remarkable, and entirely different 
from that of the ordinary augite-syenites. The feldspars are 
idiomorphic, and impress their form on the dark constituents, 
with the exception of the olivine. These latter occupy the inter- 
sticial spaces, and are penetrated by the feldspar laths in a 
manner suggestive of an ophitic structure. The character of the 
augite and hornblende, as well as the abundance of the feldspar, 
suggest a magma rich in alkalis. 
The second specimen strongly resembles the first, but in it 
the hornblende replaces the augite, and is present in large amount. 
This hornblende is so intensely colored that in many cases it is 
nearly or quite opaque, but when transparent has a deep 
