owe their origin to the causes tabulated under B5 and B6.
We find it developed as a secondary mineral in highly-silicated rocks
which have been subjected to dynamic changes ; the beginning of its
formation only commenced after many other changes had occurred in the
rock, since its crystals are attached to crystals of pre-formed
minerals, but its growth was completed before the cessation of the
movements which caused the metamorphosis; the last movements fractured
the Tourmaline crystals, chiefly transversely to their axis of greater
elongation, and bent them, and then the fissures so formed were filled
with the last of the rock-forming minerals to consolidate, since
Quartz, Felspar and Mica are found in these interstices. The absorption
of the ends of the crystals seems to have occurred at the time of the
bending; it is comparatively rarely one finds well terminated crystals,
though specimens showing the prism zones well developed and perfect are
common.
