168 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

The gneiss of the Baltimore region is penetrated with a great abund-
ance of dikes, veins and eyes of the coarse grained granite, known as
pegmatite. The other crystalline rocks of the region, although to a
less extent, contain the same materials. Within the eastern plateau
region the pegmatite appears to have been produced in two ways, at
least we seem compelled by direct evidence to assume that certain
occurrences of it are true eruptive dikes genetically related to the
normal granite already described; while for other occurrences an
aqueous origin by segregation appears more probable, although the
proof is not as good as in the former cases.

THE LATER PERIODS.

The rocks of post-Algonkian age are but poorly represented in the
eastern division of the Piedmont Plateau, and are in the main simply
outliers of those occurring in the western portion of the Piedmont
area and in the Coastal Plain. Among the more important are the
quartzites and phyllites of Paleozoic age, the Mesozoic diabase and
the superficial late Mesozoic and Cenozoic deposits.

The Paleozoic Quartzite is confined to the area of Deer Creek, in
Harford county, and is probably identical geologically with the quartz-
ites of the western division of the Piedmont Plateau. It is closely
related to them lithologically. The characteristics of the quartzite
will be discussed when the rocks of that division are considered.

The Paleozoic Phyllites, which occur as semi-crystalline slates and
schists, extend as a constantly narrowing belt from the northern bord-
ers of Cecil and Harford counties across Baltimore county into the
southeastern part of Carroll county. They probably form simply an
outlier of similar deposits found extensively developed in the western
division of the Piedmont belt and will be more fully discussed in that
connection.

The Mesozoic Diabase which intrudes the older crystalline rocks in
Baltimore and Harford counties, occurring as a long dike, broken at
several points, preserves all the features of the nominal Triassic dia-
base found in the Frederick valley.

The more recent Mesozoic and Cenozoic deposits occur as outliers