cast-iron, and will sometimes ring, on being struck, almost as
clearly. It was used very much formerly for making journals to run
mill-gudgeons upon. When found on the surface, it is usually of a
rounded form...."

However, much of the rock of the valley partakes of the nature of both
hornblend and gneiss, and has been aptly termed a "hornblend gneiss
rock."

Beds of magnesian or talcose slate, sometimes containing crystals of
sulphuret of iron, are frequently met with in this section, and at the
base of Black Oak Ridge, which is composed chiefly of chlorite slate
and epidote, another bed of magnesian limestone is found. Containing
about 40 per cent of magnesia, it makes an excellent cement for walls,
but is of little or no value as a fertilizer.


SOILS.[10]

The soils of Loudoun vary greatly in both geological character and
productiveness, every variety from a rich alluvial to an unproductive
clay occurring within her boundaries. In general the soils are deep
and rich and profitably cultivated.

The heavy clay soils of Loudoun are recognized as being the strongest
wheat and grass soils. The more loamy soils are better for corn on
account of the possibility of more thorough cultivation. However, the
lands all have to be fertilized or limed to obtain the best results,
and with this added expense the profit in wheat growing is extremely
uncertain on any but the clay soils. The loamy soils are especially