30 
GEOLOGY OF THE SECOND DISTRICT. 
blende is always in small masses; and besides, the resemblance between the two minerals is 
so great, that it always requires a careful inspection in order to distinguish them from each 
other. This variety is generally spotted, the hornblende being collected into little clusters of 
crystalline masses of a dark grey, which, appearing upon a lighter ground, gives it this aspect. 
Small grains of garnet are common in this variety ; they are often obscure, and hence require 
the assistance of a microscope to detect them. Garnet has not as yet been seen in crystals 
much larger than a pigeon shot, and these are imperfect. 
Labradorite, Hornblende and Epidote, are also of frequent occurrence ; the latter generally 
appears as a coating, which penetrates into the natural joints of the labradorite. It is known 
by its peculiar yellowish green colour. It is not in sufficient quantity to give character to 
the rock. 
Granular Labradorite and Mica, is a more common variety than the preceding ; the mass 
is quite dark, and it has more of a trappean appearance than any of the preceding varieties. 
The mica occurs in small tufted radiating masses, and almost perfectly black. It has, too, 
much of the spotted appearance of the hornblendic variety—a variety more abundant near 
the junction of the hypersthene rock with gneiss. 
Quartz, which is so abundant and constant in other primary rocks, is extremely rare in this. 
Where it occurs, it is in seams, or thin irregular veins ; indicating, as it would seem, that it 
does not form a constituent part of this rock. 
Magnetic oxide of iron is very generally diffiised through the rock ; it appears in black 
grains with a resinous lustre, and may be distinguished from the other dark colored minerals 
by the magnet. Both garnet and iron are most abundant in the rock in the neighborhood of 
the beds or veins of iron. In some instances, their presence seems to indicate proximity to 
masses of iron. 
Jointed Structure. 
Hypersthene rock is traversed by a double system of joints, in consequence of which it 
breaks into angular masses or forms. Those joints, or divisional seams, run S. 5° W. and 
N. 85° E. I have found a variation in this direction, amounting to from 5° to 10°, when 
observed at different places. Near the summit of mountains, thick beds or lamina of the rock 
separate from the mass in a direction almost parallel to the slope or face of the mountain; 
appearing precisely like stratification, and might be taken for it, did we not know that the rock 
is unstratified, and that such an arrangement would not be in accordance to the usual course 
of the layers composing a stratified mass. 
By the operation of various causes in these high regions, blocks became detached entirely 
from the main rock, and are finally perfectly insulated like ordinary boulders, and appear as 
if they had been brought from a distance, and left on the perfectly bare rock on which they 
rest; and this might perhaps be the theory which some geologists would propose to account 
for their insulated position; but being precisely similar to the mass on which they rest, it is 
more rational to consider them weather-detached masses which in ancient times once com¬ 
posed a part of the very rock on which they now repose. 
