MARYLAND GEOLOGICAL SURVEY 207

THE GRANITE AND GNEISS. —The gneiss, which has been fully
described already, is the oldest of the Maryland rocks and covers a wide
area in Cecil, Harford, Baltimore, Howard, and Montgomery coun-
ties. Through this ancient gneiss complex the granite was intruded
at a later date. The gneiss differs from the granite only in having
a more or less pronounced parallel and banded structure so that it
is not always possible to distinguish sharply between them, especially
as true granites have such a structure secondarily developed in them
by pressure.

The regions in Maryland where the granite and gneiss are most
extensively worked are at Port Deposit in Cecil county, in the vicinity
of Baltimore, at Woodstock in Baltimore county, and at Ellicott City
and Guilford in Howard county. Other areas in Howard and Mont-
gomery counties and in the District of Columbia contain some good
stone, but it is quarried only for local use.

In the extensive quarries of granite along the northern bank of
the Susquehanna river near Port Deposit, Cecil county, the rock is a
gray biotite granite-gneiss with dark colored constituents arranged in
parallel directions so as to closely resemble a gneiss. The first serious
working of the granite at this locality was in the years 1816-17, and
the business has much increased in later years. The Port Deposit rock
has afforded materials for the construction of Fortress Monroe, Forts
Carroll and McHenry, the navy yard and dry dock at Portsmouth,
Virginia; the Naval Academy at Annapolis; and many of the prin-
cipal bridges of Baltimore and Philadelphia, besides a great many
other structures both of a public and a private character.

Much granite has been quarried in the southwestern corner of Bal-
timore county near Woodstock, where a stone remarkably homogen-
eous in grain and color has been obtained. Operations were com-
menced in this area about the year 1832-33, when the large granite
boulders of the vicinity attracted the attention of practical quarry-
men. Since that time extensive quarries have been opened in this
area, the two most important known under the name of the " Wal-
tersville " and the " Fox Rock. " The rock obtained from this area
has been extensively employed in the construction of public buildings,