and that of the ordinary operations of a glass-work or iron-furnace. 
As far back as the year 1846, Scheerer observed that there exist in 
- granite various minerals which could not have consolidated save at 
a comparatively low temperature. He instanced especially gadolinites, 
orthites, and allanites, which cannot endure a higher temperature 
than a dull-red heat without altering their physical characters; — 
and he eoncluded that granite, though it may have possessed a high 
temperature, cannot have solidified from simple igneous fusion. 
We may conclude, therefore, that the confessedly igneous rocks 
of the earth’s crust, though they can be shown to have been in a 
fluid or pasty state, have not solidified from that mere simple fusion 
which we can accomplish artificially, but that conditions have been 
involved which have not been successfully imitated in any laboratory 
or furnace. We may infer also that in the modifications of rock 
structure and texture, short of actual fusion, simple dry heat has 
not been the active agent. 
Three obvious differences present themselves between the natural 
and artificial operations. (1.) The element of time must be taken 
into account; 1gneous rocks, more particularly the portions of them 
which consolidated beneath the surface, have cooled vastly more 
slowly than any artificial product. (2.) Rocks which have un- 
doubtedly once been in a liquid, others that may have consolidated 
from a pasty condition, and some which have been injected as veins 
and dykes into previously consolidated masses, contain water 
imprisoned within their component crystals. This is not water 
subsequently introduced. Ocular demonstration of the abundance 
of water in the molten magma beneath the crust is furnished by the 
enormous discharges of steam from volcanoes, and from many erupted 
lavas, long after they have congealed (p. 198). In the crystals 
of recent lava, as well as in those of early geological periods, the - 
resence of water in minute cavities may be readily detected (p. 96). 
t is contained in microscopic cells within the component minerals, 
and was enclosed with its gases and saline solutions at the time 
when these minerals crystallized out of their parent magma. 
The quartz of granite is usually full of such water-vesicles. “A 
thousand millions,” says Mr. J. Clifton Ward, “might easily be 
contained within a cubic inch of quartz, and sometimes the 
contained water must make up at least 5 per cent. of the whole 
volume of the containing quartz.” Thus microscopic investiga- 
tion confirms the conclusion arrived at by Scheerer in the memoir 
already cited, that at the time of its eruption granite must 
have been a kind of pasty mass containing a considerable 
proportion of water. It is common now to speak of the “aquo- 
igneous” origin of some eruptive rocks, and to treat their production 
as a part of what are termed the “hydro-thermal” operations of 
geology. We may conclude that, while some rocks, like obsidian 
and pitchstone, which so closely resemble artificial glasses, may have 
' Bull. Soc. Geol, France, iv. p. 468. 
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296 DYNAMICAL GEOLOGY.  [Boox IL. 
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