Granite.

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 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 one point they have been worked for commercial purposes.  Nearly every outcrop has been opened, however, for agricultural lime.  Where Goose Creek crosses this belt a quarry has been opened and good marble taken out, but want of transportation facilities has prevented any considerable development.  The relation between marble and schist is very perfectly shown at an old quarry west of Leesburg.  The marble occupies two beds in schist, and between the two rocks there is gradation of composition.  In none of the western belts are the calcareous beds recrystallized into marbles, but all retain their original character of blue and dove-colored limestone.  None of them, however, is of great thickness and none of great linear extent.