MARYLAND GEOLOGICAL SURVEY 169

upon the crystalline rocks, particularly along the eastern margin of
the plateau country. They will be severally considered in the discus-
sion of the Coastal Plain formations.

THE WESTERN DIVISION.
THE ALGONKIAN PERIOD.

The supposed Algonkian rocks of the western division of the Pied-
mont Plateau are infolded with the Paleozoic deposits of Montgomery,
Frederick and Carroll counties. They consist, so far as has yet been
observed, of but a single type, closely resembling the metamorphosed
basic volcanic rocks of the Blue Ridge district.

THE BASIC VOLCANICS. —The basic volcanic rocks represented in
Montgomery, Frederick and Carroll counties consist of long lenses or
bands infolded with the phyllites and crystalline limestones. They
follow the general structural lines in a N. E. -S. W. direction. The
lenses are more numerous in Frederick county than in the area to the
east. The rocks have been highly metamorphosed and show all the
essential characters of the " Catoctin schist" of the Blue Ridge dis-
trict and will be more fully described under that head.

THE CAMBRIAN AND SILURIAN PERIODS.

The western division of the Piedmont Plateau, comprising the larger
part of the western slope of Parr's Ridge as far as the Monocacy
river, has been described as composed mainly of semi-crystalline rocks
of sedimentary origin. These rocks are almost unaltered along their
western margin, and present the same characters as the sandstones,
slates and limestones of the Blue Ridge and Frederick valley, where
their age has been determined by fossils. As they approach the axis of
the " fan, " however, which has been shown above to be one of the prin-
cipal features in the structure of the Piedmont Plateau, these schists
become more crystalline. Here they stand nearly vertical, and show
that the dynamic action has been at a maximum by the greatly con-
torted condition of the schists and the abundant development of new
minerals within them. The slates have become roofing-slates, or