QUARTZ: ITS VARIETIES AND FORMATION. 183 



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 bub- 

 bles will be found which are movable ; the smaller ones, indeed, ap- 

 pear to be endowed with a kind of perpetual motion of their own. 

 The quartz in these rocks must have crystallized at a very high tem- 

 perature indeed, where glass cavities occur, from a state of true ig- 

 neous 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 nat- 

 urally 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 accom- 

 panying 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 w r hich 

 form some of the schistose rocks can have been produced ? We must 

 appeal to metamorphisra. The contact of highly-heated eruptive mat- 

 ter 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 



