Magnesium Amphibole. Ca Mg2(Si O4)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."
[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 valley, and renders it one of the best farming sections in Virginia.