sources of Virginia_"
prepared by the Virginia Commission to the St. Louis Exposition,
Loudoun is credited with the three comparatively rare minerals given
below. The two first-named occur nowhere else in the State.

"ACTINQLITE: _Calcium-magnesium-iron, Amphibole_,

Ca (Mg Fe)_{3}(Si O{4}){3}.

Specific Gravity, 3-3.2. Hardness, 5-6. Streak, uncolored.... Fine
radiated olive-green crystals are found ... at Taylorstown...."

"TREMOLITE: _A variety of Amphibole. Calcium._

_Magnesium Amphibole._ Ca Mg{2}(Si O{4}){3}.

Specific Gravity, 2.9-3.1. Hardness, 5.6. Long bladed crystals; also
columnar and fibrous. Color, white and grayish. Sometimes nearly
transparent. Found in the greenish talcose rocks at Taylorstown."

_Chromite_, of which no occurrence of economic importance has yet been
discovered in the County or elsewhere in Virginia.

"[9]On the eastern flank of the Catoctin rests a thin belt of mica
slate. This rock is composed of quartz and mica in varying
proportions, and this belt, on reaching the Bull Run Mountain, there
expands itself, and forms the whole base of that mountain, and where
the mica predominates, as it does there, it sometimes forms excellent
flagging stones."

[Footnote 9: Taylor's _Memoir_.]

* * * * *

"Immediately at the western base of the Catoctin Mountain, a range of
magnesian or talcose slates occur traversing its whole length.... In
this range a vein of magnesian limestone is met with, and is exposed
in several places. It however is narrow, in some places only a few
feet in thickness, and being difficult to obtain is not much sought
after for burning."

* * * * *

"Along the eastern side of the valley (Loudoun) gneiss is frequently
met with on the surface, and where the larger streams have worn deep
valleys, it is sometimes exposed in high and precipitous cliffs. This
is more particularly the case along Goose Creek and Beaver Dam.
Associated with it, however, is clay slate, not so much in rock as in
soil, for it being more readily decomposed is seldom found on the
surface, except as soil. These two varieties are often met with side
by side in thin layers, and their combination at the surface forms a
peculiarly favorable soil for agricultural purposes. The gneiss from
the quartz it contains makes a sandy soil, while the clay slate gives
it tenacity. This happy combination is a prevailing feature of this
entire vall