166 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

second case an aggregate of olivene and pyroxene with more or less
magnetite was the result. This type is called peridotite (Iherzolite).

The two non-feldspathic types of eruptive rocks, pyroxenite and peri-
dotite, are peculiarly subject to alteration, which is not, however, de-
composition. Briefly it is this: the pyroxene, when it occurs alone,
tends to pass into secondary hornblende, and this in turn gives rise to
talc. This is the origin of the extensive beds of steatite in eastern
Maryland and Virginia. The talc is always mixed with more or less
pale, fibrous hornblende (tremolite) and chlorite.

When, as in the peridotite, olivene accompanies the pyroxene,
especially if it is bronzite, the rock tends to form serpentine instead
of talc. The serpentine also contains secondary hornblende formed
from the diallage.

Both types of non-feldspathic eruptives are very intimately asso-
ciated. They do not usually cover large areas, but occur in small
lenticular patches. Varieties intermediate between the two extremes
are common, so that the two alteration products, steatite and serpen-
tine, are even more intimately mingled than the rocks themselves.

THE DIORITE. —The rocks included under this head are closely
allied to the granites and at first glance may easily be mistaken for
them. They differ, however, in the character of their feldspar and in
their darker color. They always contain a green hornblende, and
biotite with orthoclase and plagioclase, sometimes the former and
sometimes the latter in excess. Quartz is usually present and the
rocks then resemble the well-known tonalite from European localities.
Under the microscope the diorites generally show evidences of the
destruction of their constituents through dynamic action or through
weathering processes.

The areal distribution of these rocks has not been fully studied, but
they have been mapped in several small areas to the west of Wash-
ington on either side of the Potomac river. They are most exten-
sively developed, so far as recognized, around Georgetown and near
Cabin John's Bridge. At the quarries at the former place, clearly
defined inclusions of other rocks have been noticed, which substan-
tiate the view that these rocks represent ancient eruptive masses which