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.

The exhibition at the World's Fair, at New Orleans, of the following
specimens of Loudoun minerals claimed much interest from visiting
mineraloguists:

1. _Specular Iron Ore_, from near Leesburg, said to be in quantity.