160 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

semi-crystalline, and while they have been subjected to a certain
amount of metamorphism and alteration they still plainly show that
they were once sediments of an ordinary type. While as yet only a
few imperfect fossils have been found in them, they are not more
altered than similar rocks which in other localities have yielded
abundant fossil remains, so that there is good reasons to suppose that
their age may yet be definitely determined on paleontological evidence.
Although these semi-crystalline rocks are principally confined to the
western half of the plateau region, there are isolated areas of them
within the holocrystalline belt which appear to be much younger than
the rocks around them, but which have been protected from removal
by being infolded with them.

The line separating these two divisions of the Piedmont Plateau
which we shall hereafter designate as the holocrystalline (eastern) and
the semi-crystalline (western) areas, is not coincident with the crest
of Parr's Ridge, but lies on its eastern flank. Commencing in the
south near Great Falls on the Potomac it passes slightly west of
Rockville and of Hood's Mills, then to the north through Westminster
on the Western Maryland Railroad, and thence by a northeasterly
course to the Pennsylvania line. Further eastward there is a larger
area of semi-crystalline schists in Harford county surrounding the
Peach Bottom and Delta roofing-slates. These appear to be infolded
in the gneiss and are probably connected with the area near Finks-
burg by a narrow tongue passing the Northern Central Railway at
Whitehall.

The most striking feature in the structure of the Piedmont Placeau
is its radiating or fan-like structure, and the fact that the vertical strata
forming the axis of this fan follow a direction neither parallel to nor
coincident with the boundary between the crystalline and semi-crystal-
line rocks. These two lines start from the same point on the Potomac
(Great Falls), but diverge more and more toward the north. The
fan, therefore, while its axis is throughout composed of semi-crystal-
line rocks, has its western flank made up of the less crystalline and
its eastern flank of the more crystalline portion of the Piedmont region.

The different divisions in the rocks of the Piedmont Plateau are the
following: