The Weverton sandstone has suffered less from metamorphism than any of
the sediments. In the Blue Ridge it has undergone 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