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.

The Loudoun formation, of course, followed a period of erosion of the