be detected in the sandstones,
either in strike or dip, yet of course it must exist by at least the
thickness of the intrusive mass. That this thickness is considerable
is shown by the coarseness of the larger trap masses, which could
occur only in bodies of considerable size, and also by the width of
their outcrops in the westward dipping sandstones. The chief mass in
point of size is three miles wide. This mass fast decreases in width
as it goes north, without losing much of its coarseness, and ends in
Leesburg in a hooked curve. The outline of the diabase is suggestive
of the flexed trap sheets of more northern regions, but this
appearance is deceptive, since the diabase breaks directly across both
red sandstone and limestone conglomerate, which have a constant north
and south strike. An eastern branch of this mass crosses the Potomac
as a small dike and passes north into Pennsylvania. The diabase dikes
in the Catoctin Belt are always narrow, and, while many outcrops occur
along a given line, it is probable that they are not continuous.

At Leesburg the limestone conglomerate next the diabase is indurated,
its iron oxide is driven off, and the limestone partly crystallized
into marble.

_Catoctin Schist._

The Catoctin schist is geographically the most important of the
volcanic rocks of Loudoun.

Throughout its entire area the schist is singularly uniform in
appearance, so that only two divisions can be made with any certainty
at all. These are dependent upon a secondary characteristic, viz, the
presence of epidote in large or small quantities. The epidote occurs
in the form of lenses arranged parallel to the planes of schistosity,
reaching as high as five feet in thickness and grading from that down
to the size of minute grains. Accompanying this lenticular epidote is
a large development of quartz in lenses, which, however, do not attain
quite such a size as those of epidote. Both the quartz and epidote are
practically insoluble and lie scattered over the surface in blocks of
all sizes. In places they form an almost complete carpet and protect
the surface from removal. The resulting soil, where not too heavily
encumbered with the epidote blocks, is rich and well adapted to
farming, on account of the potash and calcium contained in the epidote
and feldspar.

Except along the narrow canyons in the Tertiary baselevel the rock is
rarely seen unless badly weathered. The light bluish green color of
the fresh rock