206 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

while all the flint, feldspar, kaolin and mica in the state must be
sought for in this horizon. These older or pre-Paleozoic rocks again
appear in the centre of the Blue Ridge, where they form the Middle-
town valley, and here they yield traces of copper, antimony and iron.

The Paleozoic strata which stretch from the Frederick valley west-
ward across the state, furnish much good sandstone and limestone, two
horizons of valuable cement rock, and at the top carry what is left by
man and the eroding agencies of nature of the wonderful Cumberland
Coal basin, with its 14-foot vein of solid coal. This same basin con-
tains also deposits of fire-clay and iron.

As we trace the sequence of formations through the more recently
formed portions of the state (post-Paleozoic strata of the map), we
find -them not devoid of mineral deposits of economic value. The
variegated limestone breccia, known as " Potomac marble, " and the
best brown sandstone for building purposes found in Maryland, both
belong to the oldest of these post-Paleozoic strata—the Triassic belt of
the Frederick valley and southern Montgomery county. The series
of still unconsolidated beds which represents the lapse of time from
the upper Jurassic or lower Cretaceous period to the present, and
which composes all of eastern and southern Maryland, besides fur-
nishing valuable lands for various agricultural interests, contains our
principal supply of brick, potter's and tile clay; of sand, marl and
diatomaceous earth; and much of our best iron ore.

These several economic products will be considered briefly in the
following pages.

SPECIAL DESCRIPTIONS.
THE BUILDING AND DECORATIVE STONES.

The building and decorative stones of Maryland are widely dis-
tributed throughout the central and western portions of the state, and
consist of many different varieties which by their diversity in color,
hardness and structural peculiarities are adapted for nearly all archi-
tectural and decorative purposes. Among the more important may
be mentioned the granite, gneiss, marble, limestone, slate, sandstone
and serpentine.