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°; from this they
vary up to 90° and down to 20°. 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.

Parallel with the micas that cause the schistosity, the growth of the
quartz and epidote lenses took place. These, too, have been deformed
by crushing and stretching along Bull Run Mountain and the south part
of Catoctin Mountain. From this fact, taken in connection with the
folding of the schistose planes at Point of Rocks, it would appear
that the deformation was not a single continuous effort.