474 — Nolices of Memoirs—Dr. H. C. Sorby’s Address. 
granitic type differ in a most characteristic manner from the products of artificial 
igneous fusion, both in the structure ot the crystals and in containing liquid water, 
inclosed at the time of their formation. The question then arises, whether these 
differences were due to the presence of the liquid water, or whether its presence and 
the characteristic structure were not both the effects of the great pressure of superin- 
cumbent rocks. I do not see how this can be decided in a pertectly satisfactory 
manner, but must confess that I am inclined to believe that, whilst great pressure 
was necessarily the reason why the water did not escape as vapour, the presence of 
liquid water during final consolidation must have had a very considerable influence in 
modifying the structure of the rock, and had a great share in developing what we 
may call the granitic type. 
It would be very instructive to follow out the gradual passage from one extreme 
type to another far more completely than is possible on the present occasion. ‘The 
most interesting examples of rocks intermediate between the granitic and voleanic 
types that I have been able to examine in adequate detail, are the various Cornish 
elvans and other quartz felsites, which furnish all but a complete passage from pitch- 
stone to granite. Some specimens prove that quartz may crystallize out from and 
inclose a perfectly glassy base, without a trace of liquid water; and at the same 
time other specimens prove equally well that, as we approach the granitic type, the 
quartz was not deposited from a glassy solvent, but inclosed more or less water. In 
the few intermediate cases there appears to be evidence of the conjoint presence of 
uncombined water and melted stony matter. On the whole, if we take into considera- 
tion only the external form of the larger crystals, rocks of the granitic type are very 
much as though the crystals met with in truly voleanic rocks had been strained out 
from the glassy or fine-grained base, and the intermediate spaces filled with quartz. 
The internal structure of the crystals is, however, very different, the cavities in one 
class containing glass, and in the other water. This most essential and characteristic 
difference proves that rocks of the true granitic type cannot have been formed simply 
by the more complete crystallization of the general base of the rock. If the crystals 
in granite were analogous to those developed in volcanic rocks, and the only essential 
difference were that the residue crystallized out more slowly and completely, so as to 
give rise to a more coarsely crystallized base, the crystals first formed ought not, as I 
think, to differ so essentially as that in one case they should inclose only glass, and in 
the other only water. Taking all into consideration, we can therefore scarcely suppose 
that the crystals in granitie rocks were deposited from a truly melted, dry, glassy 
solvent, like those in volcanic rocks or in slags. 
General Results.—I have, I trust, now said enough to show that the objects here 
described may be conveniently separated into three well-marked groups, viz., artificial 
slags, voleanic rocks, and granitic rocks. My own specimens all show perfectly well- 
marked and characteristic structures, though they are connected in some cases by 
intermediate varieties. Possibly such connecting links might be more pronounced in 
other specimens that have not come under my notice. I must, however, base my 
conclusions on what I have been able to study in an adequate manner, by examining 
my own preparations, and leave it for others to correct any errors into which I may 
have been led from lack of more numerous specimens. In any case the facts seem 
abundantly sufficient to prove that there must be some active cause for such a common, 
if not general, difference in the structural character of these three different types. 
The supposition is so simple and attractive, that I feel very much tempted to suggest 
that this difference is due to the presence or absence of water as a gas or as a liquid. 
In the case of slags it is ot present in any form, Considering how large an amount 
of steam is given off from erupted lavas, and that, as a rule, no fluid-cavities occur in 
the constituent minerals, it appears to me very plausible to suppose that those structures — 
which are specially characteristic of voleanic rocks are in a great measure, if not 
entirely, due to the presence of associated or dissolved vapour. The fluid-cavities 
prove that water was sometimes, if not always, present as a liguid during the con- 
solidation of granitic rocks; and we can scarcely hesitate to conclude that it must 
have had very considerable influence on the rock during consolidation. Still, though 
these three extreme types appear to be thus characterised by the absence of water, or 
by its presence in a state of vapour or liquid, I think we are scarcely in a position to 
say that this difference in the conditions is more than a plausible explanation of the 
ditferences in their structure. At the same time, I do not know any facts that are 
opposed to this conclusion, and we should, perhaps, not greatly err in thus correlating 
