The Peach Bottom slate is made up mainly of fine-grained quartz, chlorite, and muscovite.  The grains are so fine in the slate that they cannot be recognized using magnifications below 250x.  According to the Maryland Geological Survey (MGS), dusty ilmenite and small cubes of pyrite are widespread.  Traces of iron-rich biotie occur in some place; and oriented graphite composes about 2% and sometimes more in local areas.  Small chloritoid porphyblasts, not usuakk more than .1mm long, are widespread and generally oriented across the slaty cleavage.