their courses controlled by it on account of their small bulk. The
large masses break at random across the sandstone in the most
eccentric fashion. No dislocation can 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