290 VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
with depth, have been subjected to the action of water at a temperature 
of 180° C. or more. This temperature would be reached, with an incre- 
ment of one degree for 30 meters, at a depth of 5,400 meters. 
At less depths the temperature might be 180° C. or more as a conse- 
quence of igneous intrusions or other causes, in which case devitrifica- 
tion might be rapid near the surface. Indeed, in areas of regional vulcan- 
ism, and often in those of local vulcanism, the lava flows follow one 
another in such rapid succession that the beds are not cooled before they 
are buried under other lavas. However, the cooling may be sufficiently 
rapid so that the lavas solidify at least partly in the form of a glass. 
In such cases the residual heat of the lava may be sufficient, so that for 
_a long time the glass will be at a temperature above 180° C., and there- 
fore furnish conditions for rapid devitrification. 
But one must not conclude that devitrification does not occur at tem- 
peratures less than 180° C., and consequently at depths less than 5,400 
meters. Indeed, we know that unannealed glass, which cooled irregularly 
and therefore is in a state of strain—for instance, glass tubing such as is 
used in the laboratory—partly devitrifies in a few years. Moreover, even 
if the glass be not in a state of strain, there is every reason to believe that 
water at temperatures much lower than 180° C.acts upon it slowly, and 
therefore results in devitrification. Furthermore, it is probable that the 
process gocs on at a sufficient rate, so that during the millions of years 
of geolozical time complete devitrification of great masses of natural glass 
might have taken place comparatively near the surface. 
But Barus has shown that 180° C. is a critical temperature, so far as 
the solubility of glass in water is concerned, and that at temperatures 
higher than this the rate of action is increased many fold beyond the 
rate of increase of temperature. Hence we conclude, under ordinary 
conditions of temperature and mass static conditions, that recrystalliza- 
tion of glass occurs much more rapidly at depths considerably exceeding 
5,000 meters than at depths less than 5,000 meters. Since the devitrifi- 
cation of glass results both in liberation of heat and in lessening of vol- 
ume, it is concluded that this process may occur as deep as observation 
extends. 
Whether the process is slow or fast, it is certain that it has taken place 
in nature on a great scale. As evidence of this may be cited the well 
known American instances of the glass of the Original Huronian district 
described by Williams,* and the glass of South Mountain described by 
* Notes on the microscopical characters of rocks from the Sudbury mining district, Canada, by 
>. H. Williams: Ann. Rept. Geol. and Nat. History Survey’of Canada, vol. v, part F, Appendix 1, 
1893, pp. 55-82. 
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