Parallel with the micas that cause the schistosity, the growth of the quartz and epidote lenses took place.  These, too, have been deformed by crushing and stretching along Bull Run Mountain and the south part of Catoctin Mountain.  From this fact, taken in connection with the folding of the schistose planes at Point of Rocks, it would appear that the deformation was not a single continuous effort.

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 surveyor’s compass, rendering surveying extremely difficult where great accuracy is required.  In some instances the needle has been drawn as much as seven degrees from its true course.  This effect is more or less observable nearly throughout the Catoctin Mountain, and has been noted elsewhere in the County.

Chromate of iron was long ago discovered along Broad Run, and, about the same time, a bed of micaceous iron ore on Goose Creek below the Leesburg turnpike.  Copper ore is associated with the last-named mineral.

In 1860, the output of pig iron in Loudoun was 2,250 tons, and its value $58,000.  Rockbridge was the only Virginia County to exceed these figures.

In several localities small angular lumps of a yellowish substance, supposed to contain sulphur, have been found, embedded in rocks.  When subjected to an intense heat, it gives forth a pungent sulphurous odor.