This rock is composed almost entirely of the mineral pyrophyllite.   Pyrophyllite is a soft mineral (with a hardness of 1 on the Mohs hardness scale) formed by the hydrothermal alteration of volcanic deposits.  The volcanic deposits were altered as hot fluids permeated ancient ash deposits.  Pyrophyllite is mined in Hillsborough for use in high temperature ceramics (e.g. spark plugs).  The term phyllite is used to indicate that the rock is a very fine-grained, foliated metamorphic rock - similar to slate but distinguished by its sheen, which is produced by barely visible flakes of mica or other platy mineral. 

Pyrophyllite phyllite