cavities are filled up with glassy mineral matter—as, for instance, in the quartz of some of the Icelandic trachytes. Other cavities are fount!, especially in the granitic quartz, filled with gas, or sometimes with water, or liquid carbonic acid. In these latter cavities small bubbles will be found which are movable; the smaller ones, indeed, appear to be endowed with a kind of perpetual motion of their own. The quartz in these rocks must have crystallized at a very high temperature—indeed, where glass cavities occur, from a state of true igneous fusion. Mr. Sorby has shown that the solvent power of liquid water at the temperature of about 412° C. is very great: its action on glass has been such as to produce quartz-crystals from it.
There seems to be clear proof that the quartz of the granite rocks which contains partially filled fluid cavities, and cavities inclosing crystals of common salt, etc., has been formed in a partially melted mass of rock, and began to crystallize when that mass was exposed to the solvent action of liquid water, at a temperature not far below 400° C, but yet not sufficiently high to expand the water into steam. Mr. Sorby concludes that "by far the larger part of the quartz in granitic rocks was set free and crystallized through the action of liquid water, at a temperature of a dull-red heat, just visible in the dark. The exact temperature may, however, have varied considerably, since, if the pressure were not sufficiently great, the water might remain in the form of steam until the rock had cooled somewhat more." It has been noticed as somewhat remarkable that the quartz in granite should have been usually the last mineral to crystallize, although it is that one which is the most difficult to fuse, and which would therefore naturally be expected to have been solidified before the feldspar and the mica. But it has been shown that, when quartz is in combination with other mineral substances, it is often as readily fusible as they are; and thus what must be called accidental circumstances may have led, in the case of the rocks in question, to its being crystallized after the feldspar, which we so generally find to have modified the form of the quartz; this latter appearing as a glassy paste inclosing the accompanying minerals, instead of having a definite form of its own. It has also been observed that the feldspar in solidifying would liberate a sufficient quantity of heat to enable the quartz to retain its viscous state (Durocher); just as, on the other hand, in the quartz porphyries we see an instance of the analogous effect of the crystallizing quartz upon the feldspar. It is asked how the enormous masses of quartz which form some of the schistose rocks can have been produced? We must appeal to metamorphism. The contact of highly-heated eruptive matter might thus alter a quartz or sandstone into an almost pure quartz rock. Heat and pressure combined are mighty agents, which might also effect a similar change during the course of long ages.
That water at a high temperature can hold quartz in solution is well illustrated by the deposits of silicious sinter, thrown down by