555 
GRANITE AND ITS VAEIETIES. 
According to the experiments and observations of Gusta- 
vus Rose, the quartz of granite has the specific gravity of 
2*6, which characterizes silica when it is precipitated from a 
liquid solvent, and not that inferior density, namely, 2*3, 
which belongs to it when it cools in the laboratory from a 
state of fusion in what is called the dry way. By some it 
had been rashly inferred that the manner in which the con¬ 
solidation of granite takes place is exceedingly difierent from 
the cooling of lavas, and that the intense heat supposed to 
be necessary for the production of mountain masses of pla¬ 
tonic rocks might be dispensed with. But Mr. David Forbes 
informs me that silica can crystallize in the dry way, and he 
has found in quartz forming a constituent part of some tra¬ 
chytes, both from Guadaloupe ,and Iceland, glass cavities 
quite similar to those met with in genuine volcanic minerals. 
These ‘‘ glass cavities,” which with many other kindred 
phenomena have been carefully studied by Mr. Borby, are 
those in which a liquid, on cooling, has become first viscous 
and then solid without crystallizing or undergoing a definite 
change in its physical structure. Other cavities which, like 
those just mentioned, are frequently discernible under the 
microscope in the minerals composing granitic rocks, are fill¬ 
ed, some of them with gas or vapor, others with liquid, and 
by the movements of the bubbles thus included the distinct¬ 
ness of such cavities from those filled with a glassy substance 
can be tested. Mr. Sorby admits lhat the frequent occur¬ 
rence of fluid cavities in the quartz of granite implies that 
water was almost always present in the formation of this 
rock; but the same may be said of almost all lavas, and it is 
now more than forty years since Mr. Scrope insisted on the 
important part which water plays in volcanic eruptions, be-' 
ing so intimately mixed up with the materials of the lava 
that he supposed it to aid in giving mobility to the fluid 
mass. It is well known that steam escapes for months, some¬ 
times for years, from the cavities of lava when it is cooling 
and consolidating. As to the result of Mr. Sorby’s experi¬ 
ments and speculations on this difficult subject, they may be 
stated in a few words. He concludes that the physical con¬ 
ditions under which the volcanic and granitic rocks originate 
are so far similar that in both cases they combine igneous fu¬ 
sion, aqueous solution, and gaseous sublimation—the proof, 
he says, of the operation of water in the formation of granite 
being quite as strong as of that of heat.* 
When rocks are melted at great depths water must be 
present, for two reasons—First, because rain-water and sea- 
* See Quart. Geol. Jour., vol. xiv., pp. 465, 488. 
