MARYLAND GEOLOGICAL SURVEY 159

THE PIEDMONT PLATEAU.

An adequate comprehension of the crystalline rocks occurring
within the limits of the state of Maryland can only be gained through
a knowledge of the great Piedmont belt of the Atlantic border. A
brief characterization of this province must therefore precede a more
detailed description of the local geology. Along the eastern flank of
the Appalachian and Green Mountain uplifts there is an area of highly
crystalline or semi-crystalline rocks which extends from Alabama to
Maine, its northward extension reaching into the British possessions.
This zone attains its maximum width of 300 miles or more in the Caro-
linas; further north it narrows and is nearly buried beneath the Trias
in New Jersey, but beyond New York it again broadens so as to em-
brace the larger part of New England. Within this whole province
the rocks are so crystalline as to make fossils rare, while their structure
presents some of the most puzzling problems in American geology.

Many theories have obtained regarding the age of the strata of the
Piedmont belt, but it is only within very recent years that elaborate
and detailed work has begun a satisfactory solution of the mystery.
In New England the entire sequence of Paleozoic sediments is found
more or less completely metamorphosed with occasional outcrops of
more ancient crystalline rocks (Archean) showing beneath them, and
with a variety of younger. eruptive masses which have been intruded
through them. South of New York the crystalline belt acquires a
more homogeneous character both structurally and topographically,
which fact, together with its position at the eastern foot of the Appa-
lachian system, has occasioned its designation as the Piedmont Plateau.

The rocks composing the Maryland portion of the Piedmont Plateau
are divisible into two distinct classes. The members of the first class
are all completely crystalline, and whatever was their origin they
now retain no certain evidence of clastic structure, although their sedi-
mentary origin in part seems probable. The rocks of this type are con-
fined to the eastern portion of the plateau province and disappear be-
neath the overlying deposits of unconsolidated materials which com-
pose the Coastal Plain. The Piedmont rocks of the second class are