MARYLAND GEOLOGICAL SURVEY 209

THE MARBLE AND LIMESTONES. —The deposits of carbonate of lime
in the form of marble and limestone which are so abundant in the
central and western portions of the state differ widely in geological
age and lithological character, and as a result have been applied to
a variety of uses. Three types of rock are recognized: the highly
crystalline white marble of the eastern division of the Piedmont
Plateau; the fine-grained, compact and variegated varieties of the
western or semi-crystalline division of the Piedmont Plateau; the blue
fossiliferous limestones of the Appalachian Region; and the limestone
conglomerate or " Potomac marble " of Triassic age in the Frederick
valley.

The most valuable of these rocks are the highly crystalline marbles
of Baltimore county, which have an extensive development in a series
of narrow belts to the north and west of Baltimore city. The most
important of these areas is that which extends northward from Lake
Roland to Cockeysville and which is traversed by the Northern Cen-
tral Railway. Marble is quarried at Texas and to the west of Cock-
eysville, near the northern portion of this belt, but is only employed
for building purposes to any extent in the latter locality, where the
well-known Beaver Dam marble quarries have been successfully
operated for over 75 years. The rock in this locality is a finely
saccharoidal dolomite of great compactness and durability, in which
small scales of phlogopite occur in horizontal bands, representing the
original bedding of the rock. Other materials, such as quartz, tre-
molite, etc., occasionally occur. Blocks of great size can be obtained
at the quarries. The rock has been extensively used in public
structures in Baltimore, Washington and Philadelphia. Stone for the
construction of the Washington monument was taken from this
locality as early as 1814. It was also used in the construction of the
Washington monument at Washington as well as in other buildings
in that city. In Baltimore the City Hall, Maryland Club and sev-
eral churches have been made from the same material. The Drexel
and Penn Mutual Insurance buildings in Philadelphia and several
buildings in New York are also constructed in whole, or in part, of
the same material. The Texas quarries have also afforded materials
for the Belt Line tunnel.