172 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

central Frederick and Carroll counties, where the covering of sand-
stones and shales has been removed, are found penetrating the lime-
stones and phyllites. It seems probable that the dikes before referred
to as occurring in the eastern division of the Piedmont Plateau are
of similar origin. The diabase is holocrystalline and is composed
chiefly of plagioclase and pyroxene with olivene and magnetite. The
rocks penetrated have been at times considerably metamorphosed by
the molten rock, which was forced into their fissures, generally with a
hardening of the beds by partial solidification and re-crystallization.
The diabase decomposes with considerable rapidity, although the sur-
face is generally covered with large boulders of undecayed material
which show characteristic weathering.

THE APPALACHIAN REGION.

The geology of the Appalachian Region, as in the case of the
Piedmont Plateau, cannot be fully comprehended without taking into
consideration the great belt of which it forms a part. The beds of
sediments which form the limestones, sandstones and shales of the
Appalachian mountains were deposited in a wide, long trough, which
once extended from north to south throughout the region now occu-
pied by the mountains. This trough was undergoing gradual depres-
sion through most of Paleozoic time, until many thousands of feet of
conformable beds had accumulated in it, mainly as the debris of a
continental mass lying to the east.

This vast accumulation, at the close of Paleozoic time, was so com-
pressed as to be forced up into a series of great folds, forming lofty
ranges of mountains. The present Appalachians are merely the re-
mains of these ancient folds worn down by natural processes through
many successive periods. It is by no means certain that the mountain
crests ever stood higher than at present, for from the moment the
land rose above the sea the forces of denudation became active, and
with varying intensity have continued to the present day. The great
folds have been from time to time planed down to be again sculptured
as the result of elevatory movements. The compressive force which
raised these mountains acted from the east toward the west, hence the