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Notices of Memoirs—Dr. H. C. Sorby’s Address. 473 
rock when in close contact is sometimes so extremely fine-grained that it is scarcely 
crystallised, and is certainly far less crystalline and finer-grained than the artificial 
products to which I have called attention, and yet there is no passage towards those 
structures which are most characteristic of slags, or at least, no such passage as I 
should have expected if these structures depended exclusively on more rapid cooling. 
We might well ascribe something to the effect of mass, but one of my specimens 
of basalt melted and slowly cooled in a small crucible is quite as crystalline as another 
specimen taken from a far larger mass, though I must confess that what difference there 
is in this latter is in the direction of the structure characteristic of natural rocks. 
The presence or absence of water appears to me a very probable explanation of some 
differences. When there is evidence of its presence in a liquid state during the 
consolidation of the rock we can scarcely hesitate to conclude that it must’ have had 
some active influence ; but in the case of true volcanic rocks the presence of liquid 
water is scarcely probable. That much water is present in some form or other, is 
clearly proved by the great amount of steam given off from erupted lavas. I can 
scarcely believe that it exists in a liquid state, except at great depths, but it may 
possibly be present in a combined form or as a dissolved vapour under much less 
pressure, and the question is whether this water may not have considerable influence 
on the growth of crystals formed prior to eruption, before it was given off as steam. 
I do not know one single fact which can be looked upon as fairly opposed to this 
supposition, and it is even to some extent supported by experiment. M. Daubrée 
informs me that the crystals of augite formed by him at a high temperature by the 
action of water have the solid character of those in volcanic rocks, and not the 
skeleton structure of those met with in slags. The conditions under which they were 
formed were, however, not sufficiently like those probably present during the forma- 
tion of erupted lavas to justify our looking upon the explanation I have suggested as 
pete more than sufficiently plausible, in the absence of more complete experimental 
roots. 
4 Granitie Rocks.—I now proceed to consider rocks of another extreme type, which 
for distinction we may call the granitic. On the whole, they have little or nothing 
in common with slags, or with artificial products similar to slags, being composed 
exclusively of solid crystals, analogous in character only to slag-crystals of very 
different mineral nature. As an illustration, I would refer to the structure of the 
products formed by fusing and slowly cooling upwards of a ton of the syenite of 
Grooby, near Leicester. Different parts of the resulting mass differ very materially, 
but still there is an intimate relation between them, and a gradual passage from one 
to the other. The most characteristic feature of those parts which are completely 
crystalline is the presence of beautiful feathery skeleton-crystals of magnetite, and of 
long flat prisms of a triclinic felspar, ending in complex, fan-shaped brushes. There 
are no solid crystals of felspar, hornblende, and quartz, of which the natural rock is 
mainly composed, to the entire exclusion of any resembling those in the melted rock. 
As looked upon from the point of view taken in this address, the natural and artificial 
products have no structural character in common, so that I think we must look for 
other conditions than pure igneous fusion to explain the greatly modified results. We 
have not to look far for evidence of a well-marked difference in surrounding 
circumstances. The quartz in the natural rock contains vast numbers of fluid- 
cavities, thus proving that water was present, either in the liquid state or as a vapour 
so highly compressed that it afterwards condensed into an almost equal bulk of 
liquid. In some specimens of granite there is indeed clear proof that the water was 
present as a liquid, supersaturated with alkaline chlorides, like that inclosed in the 
cavities of some minerals met with in blocks ejected from Vesuvius, which also have 
~ to some extent what may be called a granitic structure. In the case of one very 
exceptional and interesting granite, there is apparently good proof that the felspar 
crystallised out at a temperature above the critical point of water—that is to say, 
at a temperature higher than that at which water can exist as a liquid under any 
pressure—and it caught up highly compressed steam, comparatively, if not entirely, 
free from soluble salts ; whereas the quartz crystallised when the temperature was so 
far lowered as to be below the critical point, and the water had passed into a liquid, 
supersaturated with alkaline chlorides, which have crystallised out as small cubes in 
the fluid-cavities, just as in the case of minerals in some of the blocks ejected from 
Vesuvius. 
Contining our attention, then, to extreme cases, we thus see that rocks of the 
