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 valley, and renders it one of the best farming sections in
Virginia.

"Another rock that is a valuable acquisition is hornblende. This kind
when first taken from the ground, is always covered as with a coat of
rust. This is doubtless the fact, for the oxydasion of the iron it
contains gives it that appearance, and colors the soil a reddish hue
in its immediate vicinity. Wherever this rock abounds, the soil is
durable and the crops are usually heavy. It is sometimes met with
having a fine grain, and so very hard as to be almost brittle, though
generally very difficult to break, and when broken strongly resembling