so as the diabase in the same section.

The areas of granite are, as a rule, long narrow belts, and vary
greatly in width.

The mineralogical composition of the granite is quite constant over
large areas. Six varieties can be distinguished, however, each with a
considerable areal extent. The essential constituents are quartz,
orthoclase and plagioclase, and by the addition to these of biotite,
garnet, epidote, blue quartz, and hornblende, five types are formed.
All these types are holocrystalline, and range in texture from coarse
granite with augen an inch long down to a fine epidote granite with
scarcely visible crystals.

_Loudoun Formation._

Among the various Cambrian formations of the Catoctin Belt there are
wide differences in uniformity and composition. In none is it more
manifest than in the first or Loudoun formation. This was
theoretically to be expected, for first deposits upon a crystalline
foundation represent great changes and transition periods of
adjustment among new currents and sources of supply. The Loudoun
formation, indeed, runs the whole gamut of sedimentary possibilities,
and that within very short geographical limits. Five miles northwest
of Aldie the Loudoun formation comprises limestone, slate, sandy
slate, sandstone, and conglomerate with pebbles as large as hickory
nuts. These amount in thickness to fully 800 feet, while less than
three miles to the east the entire formation is represented by eight
or ten feet of black slate.

The name of the Loudoun formation is given on account of the frequent
occurrence of all its variations in Loudoun County. Throughout the
entire extent of the Catoctin Belt, and especially through its central
portions, the Loudoun formation has frequent beds of sandstone,
conglomerate, and limestone. The limestones occur as lenses along two
lines; one immediately west of Catoctin Mountain, the other three or
four miles east of the Blue Ridge. Along the western range the
limestone lenses extend only to the Potomac. There they are shown on
both sides of the river, and have been worked in either place for
agricultural lime. Only the refuse of the limestone now remains, but
the outcrops have been extant until recent years. Along the eastern
line the limestone lenses extend across the Potomac and into Maryland
for about one mile, and it is along this belt that they are the most
persistent and valuable. As a rule they are altered from limestone
into marble, and at on