antial uniformity. The sections of the
Blue Ridge outcrops range around 500 feet, and those of the Catoctin
line are in the vicinity of 300. This permanent difference in
thickness along the two lines can be attributed to an eastward
thinning of the formation, thus, however, implying a shore to the west
of the Blue Ridge line. It can also be attributed to the existence of
a barrier between the two, and this agrees with the deductions from
the constituent fragments.

_Newark System._

An epoch of which a sedimentary record remains in the region of the
Catoctin Belt is one of submergence and deposition, the Newark or
Juratrias. The formation, though developed in the Piedmont plain,
bears upon the history of the Catoctin Belt by throwing light on the
periods of degradation, deposition, igneous injection, and deformation
that have involved them both.

At the Potomac River it is about 4 miles in width, at the latitude of
Leesburg about 10 miles in width, and thence it spreads towards the
east until its maximum width is, perhaps, 15 miles. The area of the
Newark formation is, of course, a feature of erosion, as far as its
present form is concerned. In regard to its former extent little can
be said, except what can be deduced from the materials of the
formation itself. Three miles southeast of Aldie and the end of Bull
Run Mountain a ridge of Newark sandstone rises to 500 feet. The same
ridge at its northern end, near Goose Creek, attains 500 feet and
carries a gravel cap. One mile south of the Potomac River a granite
ridge rises from the soluble Newark rocks to the same elevation.

As a whole the formation is a large body of red calcareous and
argillaceous sandstone and shale. Into this, along the northern
portion of the Catoctin Belt, are intercalated considerable wedges or
lenses of limestone conglomerate. At many places also gray feldspathic
sandstones and basal conglomerates appear.

The limestone conglomerate is best developed from the Potomac to
Leesburg, and from that region southward rapidly diminishes until it
is barely represented at the south end of Catoctin Mountain.

The conglomerate is made up of pebbles of limestone of varying sizes,
reaching in some cases a foot in diameter, but, as a rule, averaging
about 2 or 3 inches. The pebbles are usually well rounded, but
sometimes show considerable angles. The pebbles of limestone range in
color from gray to blue and dark blue, and occasionally pebbles of a
fi