PYROCRYSTA.LLINE ROCKS, 
57 
absence of magnesia would lead us to place them in this group, 
Epidote and titanium are common associates of hornblende, 
especially the rutile and silico-calcareous oxide of titanium. 
Hypersthene is often associated with it, but commonly with 
labradorite. It is composed of 
Silex, 
Hypersthene 
54-25 
Alumina, 
2-25 
Magnesia, 
14-50 
Lime, 
1-00 
Oxide of iron, 
24-50 
Water, 
1-00 
§ 46. Pyroxenic group. In combination with feldspar, 
pyroxene forms basalt. This rock is black and perfectly com¬ 
pact, or formed of particles which are indistinguishable by the 
naked eye. It is the most important of the group. The 
melapyres are also combinations of these minerals, to which 
may be added obsidian, pitchst.one, and peperino. These vol¬ 
canic products have but little interest in the eyes of an Ameri¬ 
can geologist. As a mineral, pyroxene is associated with, 
and hence might be grouped with, our pyrocrystalline lime¬ 
stone and serpentine. It is not common to those serpentines 
which are not associated with lime. But neither hornblende 
nor pyroxene enter into combination with limestone as a rock. 
§ 47. Limestone group. The class of limestones under con¬ 
sideration, though they contain many minerals, yet as a rock, 
it is not associated with any important ones, except serpentine 
and its congener rensselaerite. Serpentine in this grouping is 
subordinate to the limestone. Specular oxide of iron occurs in 
beds in this rock in northern New York. 
The circumstances under which this rock occurs in this 
country warrants its recognition as a rock quite as distinct from 
all others as granite. It is by no means a metamorphic mass. 
When this rock occurs among granites, it is massive and 
unlaminated; when it occurs among gneiss, mica slate, and 
hornblende rocks, it is laminated. It might perhaps be grouped 
