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