This question is one which finds its answer, it would seem, in the fact
that the sheet of mica schist included between the heavy masses of
limestone represents what at one time was possibly a bed of sandy clay
or mud, which with the metamorphosis of the region has become a
crystalline schist. The planes separating this schist and the limestone
above and that below it were evidently planes of weakness, and when the
forces which uplifted and fractured the strata exerted themselves these
rocks slipped and ground upon each other, causing considerable crushing
along these planes; possibly open crevices resulted in some portions.
Ore was subsequently deposited in these interstitial spaces, partly by
substitution of ore for limestone, no doubt, and partly by precipitation
in the crushed mass of lime and schist. The injection of a sheet of
felsite into the same plane of weakness can easily be conceived, as such
intrusive bodies always follow the lines of least resistance. The
extreme richness of the rock, together with its somewhat unusual
association of quartz and calcite, attracted no little attention to the
property at the time of its discovery.