MARYLAND GEOLOGICAL SURVEY 175

has cooled, the material has slowly crystallized, so that the glass is
now a quartz-feldspar mosaic, as the product of devitrification. In
color, when fresh, it is dark blue or gray or occasionally red, although
these colors change when the rock becomes weathered until the result-
ing tones are grayish or pinkish white. These acid volcanics have
been known under the name of quartz-porphyries, quartzites, aporhyo-
lites, etc., and have been the source of much discussion.

THE BASIC VOLCANICS. —The acid volcanics were preceded and
followed by extrusions of more basic materials which now show con-
siderable differences as the result of the varying conditions under
which they were formed, and the earth movements to which they have
been subsequently subjected. The rocks have been classed under the
head of " Andesite " and " Catoctin schist" by Keith, who has made
a special study of their occurrence. Both are affiliated in chemical
composition with the Gabbro-Andesite family. The andesite as
above distinguished is not developed in Maryland, the basic volcanics
being alone represented by the Catoctin schists. On the Potomac
river the schists seem almost crowded out by the numerous intrusions
of granite. In the Middletown valley, however, about Middletown
and to the north, as far as the state line, the schist is by far the most
prominent type of rock. The fresh exposures of this rock are light
bluish green in color and are usually covered with the schistose dull
gray or yellow slabs which arise from weathering, or by the blocks of
quartz and epidote which lie scattered over the surface after the rest
of the material has been removed. The original rock was a diabase,
which now has lost most of its characteristic features through the
metamorphism which has developed the marked schistosity. The pres-
ence of amygdaloidal structures and textural variations, combined
with the character of the field relations, makes it highly probable that
these schists represent at least two lava flows (separated by longer or
shorter intervals) which cooled slowly near the surface under condi-
tions of low pressure.

THE GRANITE. —Intimately intermingled with and cutting the pre-
ceding acid and basic volcanics is an intricately anastamosing body of
granite which occurs in long narrow belts varying in width from a