mentioned
as occurring in the neighbourhood of Diamond deposits. Other titanium
minerals, as Eutile and Sphene, may also result this way, though the
latter may be an original constituent, or may result from
thermo-metamor-phism. Should the pyroxenes contain chromium, Chromite
may be formed ; more often this is the result of hydro-metamorphism in
conjunction with the formation of Serpentine. It is almost impossible
to draw any sharp line between these various forms of metamorphism;
probably the action is always complex. Of the minerals found in
association with Diamond, many usually result from thermo-metamorphism
or from complex changes in which it bears an important part. Thus many
of the Garnets, Tremolite, Wollastonite, Sphene, Spinel, may be found
where a rock containing much calcium is acted on; in argillaceous rocks
Cyanite, Andalusite, Staurolite,and some of the Garnets may result. The
action of water under great pressure and probably at a considerable
temperature in most cases, which brings about the formation of these
minerals is at the same time changing the whole rock so that a
pegmatite or gneiss may result where the pressure is very great and is
only relieved slowly, -whereas if the pressure is somewhat less and
relief takes place suddenly an eruptive rock may result (J. G.
Goodchild, H. M. Geol. Survey., " Proc. Royal Phys. Soc." Vol. XIV.).
The theory that such rocks are the result of a regeneration of this
kind seems in accordance with the facts as observed in the field; if
so, many of the difficulties surrounding the apparently diverse origin
of Diamond become much less. In the two most reliable instances of
occurrence in situ recorded until lately—namely, at Wajra Karur and in Lapland—both observers referred
