Part I. SECT. iv. § 1.] SUBLIMATION. : 297 
been derived from a simple igneous fusion such as can be imitated in 
a furnace (though even in these the presence and influence of water 
may be traced), the vast majority of rocks have had a more complex 
origin, and in a great number of cases can be proved to have been 
mingled with more or less water while they were still fluid. Some 
of the operations of the contained water, so far as they can be inferred 
from experiment, are stated at p. 298. (8.) There can be no question 
that, in the great hypogene laboratory of nature, rocks have been 
softened and fused under enormous pressure. Besides the pressure 
due to their varying depth from the surface, they must have been 
subject to the enormous expansion of the superheated water or vapour 
which filled all their cavities, and sometimes, also, to the compres- 
sion resulting from the secular contraction of the globe and conse- 
quent corrugation of the crust. Mr. Sorby inferred that in many 
cases the pressure under which granite consolidated must have been 
equal to that of an overlying mass of rock 50,000 feet, or more 
than 9 miles, in thickness, while De la Vallée Poussin and Renard 
from other data deduced a pressure equal to 87 atmospheres (p. 97). 
It is not probable that any such thick overlying mass ever did cover 
the granite. 
If, therefore, any conclusion may be safely based upon the con- 
current testimony of experiment, it would appear that perfect anhy- 
drous fusion, or the reduction of a rock to the state of a completely 
homogeneous glass, has been a comparatively rare process in nature, 
or at least that such glasses, if originally formed, have in the vast 
majority of cases undergone devitrification and crystallization, until 
the glassy base has been reduced to a fraction of the total mass of 
the rock, or has entirely passed into a stony condition. Besides 
the obsidians and other natural glasses, traces of an original vitreous 
base can be readily observed with the microscope between the 
definitely-formed crystals of many igneous rocks. But in such 
_ rocks as granite, no glass exists, nor any trace of the crystallites so 
generally found as accompaniments of the vitreous condition. 
Doubtless such differences point to original distinctions in the kind 
and degree of fusion of the rocks. It seems reasonable to suppose 
that those rocks which show a glassy ground-mass, and the presence 
of crystallites, have been fused under conditions more nearly 
resembling those of the simple igneous fusion of experiment. 
Sublimation.—It has long been known that many mineral 
substances can be obtained ina crystalline form from the condensation 
of vapours (p. 202). This process, called Sublimation, may be the 
result of the mere cooling and reappearance of bodies which have 
been vaporised by heat and solidify on cooling, or of the solution 
of these bodies in other vapours or gases, or of the reaction of 
different vapours upon each other. ‘These operations, of such common 
occurrence at volcanic vents, and in the crevices of recently erupted 
and still hot lava-streams, have been successfully imitated by 
experiment. In the early researches of Sir James Hall on the effects 
