MARYLAND GEOLOGICAL SURVEY 163

homogeneous for considerable distances, but more usually it consists
of differently constituted layers.

The gneiss everywhere shows, in spite of a frequent persistence of
strike and dip, that it has been subjected to intense and repeated
dynamic action. This is apparent in the larger features of its
structure, in its generally crumpled, gnarled and twisted character,:
and in the profound metamorphism, amounting to almost complete;
recrystallization, which has gone on within it. No certain traces of
clastic origin have ever been detected in the Maryland gneiss, although
its sedimentary character may be inferred from its rapid alternation of
beds of different composition and from the nature of other rocks inter-
calated in it like the marble and quartz-schist.

The color of the more massive gneisses varies from white to a dark,
gray or blue. The more micaceous and hornblendic varieties are
dark brown or green. The mineral composition and structure are
quite normal for gneisses elsewhere developed. Superficial exposures
of the gneiss are very rarely fresh. This wide-spread decay extends
also for a considerable distance below the surface, at least in an incipi-
ent form, as may be seen from the very rapid disintegration in road
and railroad cuttings of rock that is sufficiently hard to require blast-
ing.

THE QUARTZ-SCHIST. —This type forms but a small portion of the
rocks of probable sedimentary origin included within the eastern
division of the Piedmont Plateau. It is more interesting from its
influence on the topography, since it causes the low ridge extending
along the south side of Green Spring and Mine Branch valleys known
as Setter's Ridge, than it is from its areal extent or even its minera-
logical composition. The quartz-schist rarely attains any consider-
able thickness, but instead seems to be closely related to the underly-
ing gneiss into which it grades by imperceptible transitions. Between
the schist and the marbles there is a sharp break, and it has been con-
sidered probable that this formation is in some way the result of
fumerole action in the gneiss. This conclusion is borne out by the
mineralogical composition. The most abundant constituent is quartz,
which occurs divided into fine beds of varying thickness by parallel