The ratios of schistose deformation in the igneous rocks are as
follows: diabase, with unstable mineral composition and small
mechanical strength, has yielded to an extreme degree; granite, with
stable composition and moderate mechanical strength, has yielded to
the more pronounced compression.


MINERAL AND KINDRED DEPOSITS.

In point of mineral wealth Loudoun ranks with the foremost counties of
the State. Iron, copper, silver, soapstone, asbestos, hydraulic
limestone, barytes, and marble are some of the deposits that have been
developed and worked with a greater or lesser degree of success.

A large bed of compact red oxide of iron lies at the eastern base of
the Catoctin Mountain, on the margin of the Potomac River. Long before
the Civil War a furnace was erected here by Samuel Clapham, Sr., for
the reduction of this ore, and considerable quantities of it were
formerly transported moderate distances to supply other furnaces. The
Clapham furnace continued in operation until all the fuel at hand was
consumed and then went out of blast. Water power was supplied by the
Catoctin Creek, which flows into the river immediately above the
mountain. To obtain this a tunnel was cut through a spur of the
mountain projecting into a bend of the creek. This tunnel, about five
hundred feet long and sixty feet beneath the summit of the hill, was
cut through almost a solid wall of rock, and, at that day, was
considered a great work.

Magnetic iron ore has been found in certain places, and this or a
similar substance has a disturbing effect upon the needle of the