292 THE COASTAL PLAIN OF NORTH CAROLINA.

igneous origin, the former so altered as to no longer preserve the re-
mains of organisms by which their age may be determined, while over-
lying these in places were Triassic sandstones, shales, conglomerates,
and igneous rocks in the district to the west of the present Coastal Plain.

The base-leveled condition of this Piedmont region at the opening
of the Cretaceous had permitted the deep decay of the rocks and re-
moval by percolating waters of a large part of their more soluble con-
stituents, as a result of which the remaining less easily soluble quartz,
kaolin, and mica were rendered in a fit condition for rapid mechanical
removal.

The large content of silica in the form of quartz grains, as a rule
quite angular, which probably does not fall below 25 per cent even in
the purest clays,1 the usual large percentage of kaolin grains whose
angularity in many instances shows them to have been deposited shortly
after their removal from their source without subsequent reworking,
and the common occurrence of muscovite mica as’a rule fairly clear
and giving a good biaxial figure, indicates that the source of the oldest
Coastal Plain materials must have been, at least in large part, highly
siliceous crystalline rocks.

When the causes inaugurating Coastal Plain deposition are consid-
ered it is evident that by a tilting of the land surface to the southeast-
ward the Piedmont border was submerged and the region to the north-
west was sufficiently elevated to set in motion the erosive forces of the
streams, which immediately began to entrench themselves and to trans-
port the products of the disintegration of the Piedmont crystalline rocks
to the submerged region to the southeast. Whether this submergence
was due to a warping which produced a long northeast-southwest
trough, constituting a shallow inland sea or sound more or less com-
pletely shut off from the main Open sea by land barriers to the east, or
whether it was a shallow sea margin, is a question that cannot be sat-
isfactorily answered. The lithologic nature of the deposits would seem
to favor the former view. The absence of marine faunas and the
presence chiefly of reptilian remains and fossil leaves would seem also
to strengthen this interpretation. There is also evidence of a land
_ barrier of some kind in the region about Wilmington in southeastern
North Carolina as indicated by the absence there of beds representing
_the Patuxent formation, or in fact any sediments of the Lower Creta-
ceous, for a deep-well boring showed what are probably Upper Creta-
ceous beds resting directly upon basal granite at a depth of 1,109 feet.

_

"See Bull. No. 13, N. C, Geol. Survey.