ompass, 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.

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, asso