MARYLAND GEOLOGICAL SURVEY 185

stone which is seldom coarse-grained, although in a few instances
slightly conglomeratic. Thin layers of black shale and coaly streaks,
in which plant remains are sometimes preserved, occur in some locali-
ties, although not a conspicuous feature of the formation. The sand-
stones have afforded good flagging materials. The thickness of the
formation varies from 100 to 200 feet, but the deposits are seldom
well exposed. The sandstone is, however, something of a factor in
the topography, and usually forms a line of foothills along the flanks
of the mountains.

THE GREENBRIER FORMATION. —The Greenbrier formation, so called
from Greenbrier county, West Virginia, occurs in very much the same
areas in western Allegany and Garrett counties as above described
for the Pocono sandstone, and outcrops above the line of foothills just
described. The deposits consist mainly of limestone strata in which
are interbedded shales and some sandstones. The limestones are
more sandy towards the base. The limestones near the upper portion
of the formation are of compact structure and gray in color. They
are also at times marly in their upper layers and these marly strata are
frequently fossiliferous. The limestone is burned locally for building
and agricultural purposes. The formation has a thickness of about
200 feet.

THE MAUCH CHUNK FORMATION (Canaan formation). —The Mauch
Chunk formation, so called from Mauch Chunk, Pennsylvania,
flanks the ridges of western Allegany and Garrett counties and grades
gradually downward into the Greenbrier deposits. The strata con-
sist chiefly of red shales interstratified with flaggy, red-brown, fine-
grained sandstones. The sandstone is at times micaceous. Thin
beds of dark carbonaceous shales occur at times near the top of the
formation. The deposits have a thickness of from about 800 to 1000
feet.

THE POTTSVILLE FORMATION (Blackwater formation). —The Potts-
ville formation, so called from Pottsville, Pennsylvania, is the lowest
division of the Coal Measures and forms the mountain ridges which
border the coal basins. The Pottsville formation consists of beds of
sandstone and conglomerate interstratified with sandy shales in which
thin beds of coal are locally developed. The sandstones and con-