The granite in the southern portion of the County is very important in
point of extent, almost as much 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