HYPERSTHENE ROCK. 
75 
to it. The characteristics of the rock are derived from the 
labradorite. Hornblende and pyroxene both replace the hyper- 
sthene at certain localities. This rock, when changed by the 
action of the weather? becomes light colored, and resembles the 
g’ray granites. The atmosphere acts upon the rock in stripes 
or bands, which run in the direction of the natural joints. 
The action of the atmosphere, rains? and frosts is as great as 
upon any of the known granites. Upon the tops of the 
Adirondacks large masses are strewed over the surface like 
huge boulders, but still in situ having been quarried out by 
atmospheric agencies alone. 
There are but few varieties of hypersthene rock which are 
worthy of special notice, of which the following are the most 
important; 
1. The first is composed wholly of labradorite, though to the 
eye it has the aspect of being made up of two distinct minerals. 
2. Labradorite and hornblende. The hornblende takes the 
place of hypersthene, though sometimes the latter is still 
present. 
3. Labradorite, hornblende, and epidote. The latter, how¬ 
ever, never occurs insufficient quantity to change the character 
of the rock. 
4. Granular labradorite and mica. This variety is quite 
dark, and resembles a trap. The mica is in tufted, radiated 
masses, and almost black. It occurs usually at the junction of 
the rock with gneiss. 
Magnetic iron in grains is diffused or disseminated through 
the rock. It is black, with a resinous luster. Its obedience to 
the magnet serves to distinguish it from other dark-colored 
minerals. 
Hypersthene rock is traversed by a double system of joints, 
in consequence of which it often appears in the process of 
separating into large tabular masses. One set of joints run S. 
5° W. The separation of tabular masses is frequently in the 
direction of the slope or side of the mountain where the rock 
occurs. A separation of the masses also often takes place in 
