PRECIOUS STONES.	125
	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