162 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

are not the only ones which belong to these rocks, but that their pres-
ent metamorphism and complexity must be accounted for by assum-
ing that they have been subjected to several successive periods of dis-
turbance.

The rocks composing the holocrystalline portion of the Piedmont
Plateau in Maryland are petrographically divisible into seven distinct
types. Four of these are of undoubtedly eruptive origin and may
be designated according to their chemical and mineralogical composi-
tion as diorite, gabbro, peridotite or pyroxenite, and granite. The re-
maining types through which the eruptive rocks have broken—gneiss,
marble and quartz-schist—are completely crystalline, and therefore
exhibit no certain trace of clastic structure, although it seems highly
probable that they were of sedimentary origin. Since all four types
of eruptive rocks have broken through and more or less modified the
other rocks, they are younger than the latter. The intense dynamic
action which has produced such recrystallization in the gneiss complex
has likewise greatly metamorphosed the eruptive rocks, and yet not
enough to obliterate their original character. Each type exhibits
several chemical and structural facies dependent upon the original
differentiation of the magma or upon the conditions of solidification,
to which must be added other variations due to subsequent metamor-
phism.

THE GNEISS. —The prevailing rock of the entire holocrystalline
area is the gneiss. It enters the state from the north in a very wide
band, completely surrounding the Delta Peach Bottom slate area, but
its breadth rapidly contracts toward the Potomac. The remarkably
irregular form of the marble areas which are intercalated in the gneiss
complex shows how intricate the stratigraphy of the latter really is.
Much of its apparent simplicity is due to the obliteration of its true
bedding through secondary foliation. The Maryland gneiss embraces
a great variety of types, which range from granitoid aggregates of
feldspar and quartz on the one hand to nearly pure mica or hornblende
schist on the other. All of these also show considerable structural
variation in the coarseness of their grain, the perfection of their
parallel arrangement, etc. The gneiss is sometimes quite constant or