218 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

glauconitic. The proportion of shells is often very great, so that the
Miocene marls are commonly known under the term of " shell marls. "
They occur abundantly in Queen Anne's, Talbot, Calvert and St.
Mary's counties, but have never been used so largely as the green-
sand marls which lie to the north of them.

THE IRON ORES.

The iron industry in Maryland was developed early in colonial
days and continued until a recent date to be one of the most important
factors in the prosperity of the state. Numerous references are found
in the earlier records to the iron ores, and early in the 18th century
we find considerable activity in the manufacture of iron. The Prin-
cipio Company, one of the most important commercial enterprises in
the early days of Maryland, was organized in 1722 and began the
erection of a furnace in Cecil county near the mouth of Principio
creek. In 1761 the Governor and Council of Maryland reported to
the Commissioners of the Board of Trade and Plantations in England
that there were eighteen furnaces and ten forges in the state which
made 2500 tons of pig iron per year. During the Revolutionary
War the furnaces and forges of the Principio Company supplied bar
iron and cannon balls to the Continental Army. The Principio Com-
pany during the war of 1812 produced cannon, cannon balls and
hardware, and guns as large as 32 pounders were made for the govern-
ment. Many furnaces were built in other sections of the state
during the 18th and early portion of the present century, but nearly
all of them have been abandoned. Among the most important of
these furnaces is the Catoctin furnace in Frederick county, which
was built in 1774 and furnished guns and projectiles to the Conti-
nental Army during the Revolutionary War. In more recent years
several modern furnaces have been constructed near Baltimore, of
which by far the largest is that at Sparrow's Point, built by the
Maryland Steel Company, which, however, only employs ore obtained
from sources outside of the state.

The only furnaces now manufacturing Maryland iron to any extent
are the Muirkirk furnace in Prince George's county and the Stickney