MARYLAND GEOLOGICAL SURVEY 161

. ROCK DIVISIONS OF THE PIEDMONT PLATEAU.

MESOZOIC Rock Divisions of the Piedmont

Plateau
Triassic............................. Diabase.

Newark Formation.
PALEOZOIC.

Silurian and Cambrian.............. Phyllite and Crystalline Limestone.

Quartzite.
ARCHEAN.

Algonkian and Archean (?)........... Granite.

Diorite.

Basic Volcanics.

Peridotite and Pyroxenite.

Gabbro.

Marble.

Quartz-schist.

Gneiss.

THE EASTERN DIVISION.
THE ARCHEAN AND ALGONKIAN PERIODS.

The formations of supposed pre-Cambrian age, which compose the
eastern or holocrystalline division of the Piedmont Plateau, cross
Maryland from the southeast corner of Pennsylvania and the northern
end of Delaware in a general southwest direction. Their course, how-
ever, is not a straight one through the state, but forms a double curve
whose south side is convex on the east and concave on the west. This
curve corresponds to the great westerly bend in the course of the
Triassic sandstone and folded Paleozoic bands of eastern Pennsyl-
vania. It is much less distinct in the highly crystalline rocks of the
eastern Piedmont region, but that its presence can be traced at al]
amid the varied and complex structures of these very ancient rocks is
welcome evidence that at least the final impression was imparted to their
strike by the great Appalachian folding. The convex or eastern branch
of this curve may be most distinctly traced in the belt of marble
north of Baltimore, which, near Towsontown, turns from a south-
west direction to a trend directly west through the Green Spring val-
ley. Toward the southwest these same marble belts turn again to the
south-southwest, as do all the other rocks with which they are asso-
ciated, and this course is held into Virginia. There is abundant evi-
dence that these structural features of the eastern Piedmont region