ndergone no greater change
than a slight elongation of its particles and development of a little
mica. Along Catoctin Mountain, from the Potomac River south, however,
increased alteration appears together with the diminution in
thickness. What little feldspar there was is reduced to quartz and
mica, and the quartz pebbles are drawn out into lenses. Deposition of
secondary quartz becomes prominent, amounting in the latitude of Goose
Creek to almost entire recrystallization of the mass. A marked
schistosity accompanies this alteration, and most of the schistose
planes are coated with silvery muscovite. Almost without exception
these planes are parallel to the dip of the formation.

Metamorphism of the Loudoun formation is quite general. It commonly
appears in the production of phyllites from the argillaceous members
of the formation, but all of the fragmental varieties show some
elongation and production of secondary mica. The limestone beds are
often metamorphosed to marble, but only in the eastern belt. The
recrystallization is not very extensive, and none of the marbles are
coarse grained.

The metamorphism of the igneous rocks is regional in nature and has
the same increase from west to east as the sediments.

In the granite it consists of various stages of change in form,
attended by some chemical rearrangement. The process consisted of
progressive fracture and reduction of the crystals of quartz and
feldspar, and was facilitated by the frequent cleavage cracks of the
large feldspars. It produced effects varying from granite with a rude
gneissoid appearance, through a banded fine gneiss, into a fine quartz
schist or slate. These slaty and gneissoid planes are seen to be
parallel to the direction and attitude of the sediments, wherever they
are near enough for comparison.

Dynamic alteration of the Catoctin diabase is pronounced and
wide-spread. Macroscopically it is evident in the strong schistosity,
which is parallel to the structural planes of the sediments when the
two are in contact. In most areas this alteration is mainly chemical
and has not affected the original proportions of the rock to a marked
extent. Its prevalence is due to the unstable composition of the
original minerals of the rock, such as olivine, hypersthene, and
pyroxene. Along Catoctin Mountain, however, both chemical and
mechanical deformation have taken place, so that the original rock
structure is completely merged into pronounced schisto