74 
AMERICAN GEOLOGY. 
Augusta counties, forming one of the rocks of the Blue ridge, 
where it is associated with gneiss and granite. At this place 
it is a dark gray porphyritic rock, and contains epidote. In 
North Carolina it forms a wide belt, running northeast and 
southwest in Randolph county, and on towards the narrows of 
the Yadkin. It is a black tough rock, in which hornblende 
predominates, and in which quartz is only sparsely scattered 
through it. 
The rock quarried at Quincy, Cape Ann, and at other places 
in the eastern part of Massachusetts, and which is so well 
adapted for columns and the walls of buildings, is more closely 
allied to the true granites. These are not tough and difficult 
to be quarried, because the hornblende is never in excess. 
To the foregoing varieties there might be added a pyroxenic 
sienite—a kind in which pyroxene forms a perceptible part 
of the rock. Sienite is more closely related to greenstone than 
to the ordinary granites, and it often passes into the former rock. 
HYPERSTHENE ROCK. 
§ 63. This rock is regarded as a granite. In constitution it 
differs from the common granites in being composed of labra- 
dorite, feldspar, and hypersthene, the last of which is allied to 
hornblende. The feldspar contains lime and soda. Its compo¬ 
sition has already been given. The color of this rock is usually 
a smoke gray. The color of the labradorite, however, deter¬ 
mines the color of the rock. It has the usual granitoidal 
structure. This results from the crystallization of the feldspar, 
a portion of which is fine, and represents the base, in which 
there is imbedded individuals of a large cleavable size. These 
cleavable individuals present very frequently a beautiful opal¬ 
escence of bronze, yellow, blue, and green colors. In the 
mechanical arrangement of its particles of composition it 
resembles a porphyry; but the rock chemically considered con¬ 
sists mostly of labradorite, the hypersthene being extremely 
rare in it. The rock is destitute of mica, and almost of quartz; 
and if quartz and common feldspar occur, they are subordinate 
