MARYLAND GEOLOGICAL SURVEY 171

subjected, becomes so great that it is not always easy to distinguish
the line of contact between them and the underlying and more ancient
crystallines of the eastern Piedmont region. The recent discovery of
fossils in the phyllites east of Araby by Mr. Keith proves some of
these rocks to be of Cambrian age.

THE TRIASSIC PERIOD.

The rocks of Triassic age are mainly confined to the western margin
of the Piedmont Plateau and are represented by both sedimentary
and eruptive materials which will be further described under the head
of the Newark Formation and the Diabase.

THE NEWARK FORMATION. —The deposits of the Newark forma-
tion unconformably overlie the limestone and phyllite which have
been above described and cover a considerable area along the western
border of the Piedmont Plateau. Beginning as a belt some ten miles
in width in northern Carroll and Frederick counties, the formation
gradually narrows toward the south, until in the region of Frederick
its full width does not exceed one mile, while at one point directly
to the west of Frederick the continuity of the beds is completely
broken. Farther southward in western Montgomery county the belt
of Newark deposits again broadens to a width of several miles.

The rocks of the Newark formation consist largely of red and gray
sandstones and conglomerates of both silicious and calcareous varieties.
The finer grained and deeper colored deposits generally have their
individual elements united by a ferruginous cement, while the cal-
careous conglomerate, which, is largely made up of rounded lime-
stone pebbles, is generally imbedded in a reddish calcareous matrix.
All of the deposits present structures which indicate that they were
formed in shallow water; the coarse conglomerates, the ripple-marked
surfaces, and the tracks of animals all point indisputably to this con-
clusion.

THE DIABASE. —The sandstones and shales of the Newark forma-
tion, as well as the rocks of earlier age, are found penetrated by dikes
of an igneous rock known as diabase. These dikes extend across the
area, for the most part, in a north-south direction, and throughout