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 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 schistosity.  This was materially assisted by the weak lath shapes of the feldspar and the mobility of the micas.

The average dip of the schistose planes is about 60 deg.; from this they vary up to 90 deg. and down to 20 deg..  In all cases they are closely parallel to the planes on which the sediments moved in adjustment to folding, namely, the bedding planes.  In regions where no sediments occur, the relation of the schistose planes to the folds can not be discovered.