changes on exposure to a dull gray or yellow, and the
massive ledges and slabs split up into thin schistose layers. It is
quite compact in appearance, and as a rule very few macroscopic
crystals can be seen in it.

A general separation can be made into an epidotic division
characterized by an abundance of macroscopic epidote and a
non-epidotic division with microscopic epidote. These divisions are
accented by the general finer texture of the epidotic schist.

The schists can be definitely called volcanic in many cases, from
macroscopic characters, such as the component minerals and basaltic
arrangement. In most cases, the services of the microscope are
necessary to determine their nature. Many varieties have lost all of
their original character in the secondary schistosity. None the less,
its origin as diabase can definitely be asserted of the whole mass. In
view of the fact, however, that most of the formation has a well
defined schistosity destroying its diabasic characters, and now is not
a diabase but a schist, it seems advisable to speak of it as a schist.

Sections of the finer schist in polarized light show many small areas
of quartz and plagioclase and numerous crystals of epidote, magnetite,
and chlorite, the whole having a marked parallel arrangement. Only in
the coarser varieties is the real nature of the rock apparent. In
these the ophitic arrangement of the coarse feldspars is well defined,
and in spite of their subsequent alteration the fragments retain the
crystal outlines and polarize together. Additional minerals found in
the coarse schists are calcite, ilmenite, skeleton oblivine, biotite,
and hematite.

_Rocks of the Piedmont Plain._

The Piedmont plain, where it borders upon the Catoctin Belt, is
composed in the main of the previously described Newark strata, red
sandstone, and limestone conglomerate. East of the Newark areas lies a
broad belt of old crystalline rocks, whose relations to the Catoctin
Belt are unknown.

The rocks, in a transverse line, beginning a little to the east of
Dranesville, in Fairfax County, and extending to the Catoctin
Mountain, near Leesburg, occur in the following order, viz: Red
sandstone, red shale, greenstone, trap, reddish slate, and
conglomerate limestone.

Heavy dykes of trap rock extend across the lower end of the County,
from near the mouth of Goose Creek to the Prince William line. "These,
being intrusive rocks, have in some places displaced the shale