CURRENT PRE-CAMBRIAN LITERATURE 533 
form the eastern or holocrystalline division of the Piedmont Plateau 
region of Maryland, crossing the state in a general southwest direction 
from the southeast corner of Pennsylvania and the north end of Dela- 
ware. ‘These rocks are but a part of the great crystalline plateau which 
extends from New York to Alabama along the eastern base of the 
Appalachians. Towards the east the pre-Cambrian rocks of Maryland 
plunge under Coastal Plain deposits, and toward the west they form 
the floor to support the Paleozoic strata of the Appalachians, reappear- 
ing in the granitic and volcanic rocks of South Mountain of Pennsy|- 
vania. The holocrystalline rocks are divisible into six types, three of 
which, gabbro, peridotite or pyroxenite, and granite, are of undoubted 
eruptive origin, and three of which, gneiss, marble, and quartz-schist, 
while showing no certain evidence of clastic structure, are believed to 
be sedimentary. ‘The prevailing rock is gneiss, closely associated with 
marbles and quartz-schists, forming an intricate complex. The complex 
shows evidence of great dynamic action, the rocks, having been almost 
completely recrystallized. The eruptive rocks are all younger than the 
gneisses. The gabbro is the oldest, followed by the peridotite or 
pyroxenite, and the youngest is the granite. The granites are as a 
rule medium-grained biotite-granites, but they frequently take the form 
of pegmatite. 
Williams‘ considers the general relations of the granitic rocks in 
the Middle Atlantic Piedmont Plateau and maps the same. The 
criteria by which ancient plutonic rocks in highly metamorphosed 
terranes may be recognized comprise radiating dikes, inclusions of 
fragments, contact zones, chemical composition, and petrographical 
structure. On these criteria it is concluded that most of the granitic 
rocks of Maryland are igneous, although many of them are changed to 
granite-gneiss, and of certain of these gneisses it cannot be asserted 
whether they are of aqueous or of igneous origin. South of Laurel, 
in the large area from Triadelphia southward to Brookville, at Murdoch 
Mill west of Washington, south of Falls Church in Fairfax county, Va., 
and at Cabin John Bridge on the Potomac River, there are gradations 
between granitic rocks and diorites or gabbros. In the Maryland 
General relations of the granitic rocks in the Middle Atlantic Piedmont 
Plateau, by GEorGE H. WILLIAMS. Introduction to Origin and Relations of Central 
Maryland Granites, C.R. Keyes, Fifteenth Ann. Rept., U. S. Geol. Surv., 1895, pp. 
659-684, Pls. XXVII-XXXV. 
