MARYLAND GEOLOGICAL SURVEY 225

in Frederick county, where it occupies a considerable area near the
northern border of the state in the vicinity of Emmitsburg. The
chief cementing material in all of the igneous rocks is the hydrous
oxide of iron.

The next group of road-building materials includes the marble,
the limestone and the calcareous sandstones and shales. The carbonate
of lime contained in these deposits acts as a valuable cement, but the
materials have far less durability than the igneous rocks above de-
scribed. They are found covering widely separated areas throughout
the Piedmont Plateau and Appalachian Region, the most extensive
and available deposits being found in the long, narrow valleys to the
north of Baltimore city and in the Frederick and Hagerstown valleys
farther west. These materials have already been considerably em-
ployed for road-building purposes.

The third group of road-building materials includes the gravels of
the eastern and southern portions of the state, which belong to the
late Mesozoic and Cenozoic formations. They cover extensive areas
in Cecil, Kent, Queen Anne's, Talbot, Anne Arundel, Calvert, Prince
George's and Charles counties, and with lessening importance extend
into the more southern portions of the state. These gravels are rich
in iron, which acts as the cementing material. They probably afford
less permanent road metal than the igneous rocks which were first
described, but when properly used are of great value in road con-
struction.

Several of the other rocks, both in the Piedmont Plateau and the
Appalachian Region, have been locally employed for road-building
purposes, some of the schists and shales as well as some of the more
quartzose rocks proving of value under certain conditions, but none of
them have the valuable cementing qualities of the three groups of
rocks above described.

THE MISCELLANEOUS PRODUCTS.

There are several other mineral substances of greater or less econo-
mic importance, which are either being worked to-day to some extent
in Maryland or which have been earlier worked within the state, in