216 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

near the bottom of the coal measures and the clay bed ranges from S
to 20 feet in thickness. It is divided into two varieties, designated
as the hard and soft clay. The hard clay is of a gray color, shading
almost to black; it is non-plastic, unless ground into an impalpable
powder, and disintegrates but little upon exposure to the weather.
The soft clay is very plastic, much lighter in color and crumbles
rapidly under atmospheric influences. The impurities in this clay are
fewer and smaller in amount than in most other fire clays. The two
most valuable characteristics of this clay are its freedom from potash
and the large proportion of silica to alumina.

The clay industry has grown rapidly in importance during recent
years, and the value of the output during 1896 was $1, 753, 003.

THE SANDS.

The sand deposits of the state are widely extended both in the east-
ern and western sections, but have been but little developed hitherto.
The sandy sediment which has been deposited upon the bottom of the
Potomac river has been dredged in recent years and used extensively
for building purposes in Washington.

The most important sand deposits in the eastern portion of the state
are found in the Raritan formation in Anne Arundel county, and
extensive openings have been made near the head of the Severn river,
where a very pure grade of glass sand is taken out. The output of
these diggings is transported on small schooners which are able at high
tide to reach the head of the river.

The Tuscarora (Medina) and the Monterey (Oriskany) formations
of the western portion of the state also afford very pure deposits of
quartz which have been ground up and employed to some extent in
glass-making.

The sandstones both in the eastern and western sections of the
state are capable of much further development. The output from
these formations during the year 1896 had a value of only $1, 752.

THE PORCELAIN MATERIALS.

The state of Maryland is well provided with porcelain materials.
The three principal requisites in the manufacture of porcelain are
flint (vein quartz), feldspar, and china clay (kaolin).