Name:  Gray Slate

Geologic Name:  Bangor Slate (Martinsburg Shale)

Reference:  Behre, C.H., Jr., Slate in Pennsylvania: Pennsylvania
Geol. Survey, 4th ser., Bull. M16, p. 199-223, 1933.

Microscopic characters:  In ordinary light the slide is clouded with
fine dust-like particles, which in places are segregated into black
non-transparent patches.  This is probaly graphite.  Between crossed
nicols there is a background of extremely fine, low birefringent
material thickly dotted with small grains of minerals with high
interference colors.  This is largely calcite and sericite.  A few
small grains of magnetite and quartz are apparent.