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.
Small quantities of silver ore are discovered from time to time; but the leads have never been extensively worked and many of the richest veins are still untouched.
Deposits of copper in the schists have long excited interest and led to mining operations. The amount of ore, however, appears not to have justified any considerable work.
Near the base of the Catoctin Mountain, where it is first approached by Goose Creek, marble of an excellent quality is found but has been little worked. Among the varieties at the quarry are included pure white, white and pink, blue and white, white and green, serpentinized and chloritic serpentinized marble. These marbles are of great beauty and susceptible of a good polish. The calcareous bed here is about fifty feet thick and reaches southward for three miles with increasing thickness. At its southern end it is not entirely metamorphosed into marble, but retains its original character of fine blue limestone. Northward along this range the thickness of the marble constantly diminishes and rarely exceeds ten feet. Sometimes there are two beds, sometimes only one. At Taylorstown, just south of the Potomac, the bed is about three feet thick; on the north side of the Potomac about four or five feet. Here, as elsewhere, the beds of marble are inclosed in a bluish green micaceous schist, which has been thoroughly transformed by mechanical pressure.
In the vicinity of Leesburg and north of that town, and between the Catoctin Mountain and the Potomac River, the conglomerate limestone or brecciated marble is found in abundance, associated with red shale. It is a calcareous rock, apparently formed in part of pebbles cemented together and, when burned, produces an inferior lime. It is commonly known as Potomac marble. Of this variegated marble were formed the beautiful columns in the old Representatives' chamber of the Capitol at Washington. The soil in which this rock occurs is extremely productive and valuable.