a i. ans On ee aes 
- P ; ‘ 
’ Notices of Memoirs—Dr. H. C. Sorby’s Address, 469 
and erupted rocks. In making this choice I was also influenced by the fact that in 
my two anniversary addresses as President of the Geological Society I have recently 
treated on the structure and origin of modern and ancient stratified rocks, and I felt 
that, if in the present address I were to treat on certain peculiarities in the structure 
of igneous rocks, I should have described the leading conclusions to which I have 
been led by studying the microscopical structure of nearly all classes of rocks. It 
would, however, be impossible in the time now at disposal to treat on all the various 
branches of the subject. Much might be said on both the purely chemical and purely 
mineralogical aspects of the question ; but though these must not be ignored, I pro- 
pose to draw your attention mainly to another special and remarkable class of facts, 
which, so far as I am aware, have attracted little or no attention, and yet, as I think, 
would be very instructive if we could fully understand their meaning. Here, how- 
ever, as inso many cases, the observed facts are clear enough, but their full significance 
somewhat obscure, owing to the want of adequate experimental data or sufficient 
knowledge of general physical laws. 
A considerable amount of attention has already been paid to the mineral constitu- 
tion of slags, and to such peculiarities of structure as can be learned independently of 
thin microscopical sections. A very complete and instructive work, specially devoted 
to the subject, was published by Von Leonhard about twenty-two years ago, just at 
the time when the microscope was first efficiently applied to the study of rocks. Since 
then Vogelsang and others have described the microscopical structure of some slags in 
connexion with their studies of obsidian and other allied volcanic rocks. At the date 
of the publication of Von Leonhard’s work the questions in discussion differed 
materially from those which should now claim attention. There was still more or 
less dispute respecting the nature and origin of certain rocks which have now been 
proved to be truly volcanic by most unequivocal evidence; and I am not at all sur- 
prised at this, since, as I shall show, there is such a very great difference in their 
characteristic structure and that of the artificial products ot igneous fusion, that, but 
for the small portions of glass inclosed in the constituent crystals, described by me 
many years ago under the name of ‘ glass-cavities,’ there would often be no positive 
proof of their igneous origin. There was also considerable doubt as to the manner 
in which certain minerals in volcanic rocks had been generated. The observed facts 
were sufficient to prove conclusively that some had been formed by sublimation, others 
by igneous fusion, and others deposited from more or less highly heated water ; but it 
was difficult or impossible to decide whether in particular cases certain minerals had 
been formed exclusively by one or other process, or sometimes by one and sometimes 
by the other, or by the combined action of water anda very high temperature. I 
must confess that, even now that so much may be learned by studying with high 
magnifying powers the internal structure of crystals, I should hesitate very much in 
deciding what were the exact conditions under which certain minerals have been 
formed. This hesitation is probably as much due to inadequate examination and to 
the want of a complete study of typical specimens, both in the field and by means of 
the microscope, as to the unavoidable difficulties of the subject. Such doubt, how- 
ever, applies more to the origin of minerals occurring in cavities than to those con- 
stituting a part of true rock-masses, to which latter [ shall almost exclusively refer 
on the present occasion. In the formation of these it appears to me that sublimation 
has occurred to a very limited extent. In many cases true igneous fusion has played 
such a leading part that the rocks may be fairly called igneous, but in other cases, 
water, in some form or other, has, I think, had so much influence that we should 
hesitate to call them igneous, and the term erupted would be open to far less objection, 
since it would adequately express the manner of their occurrence, and not commit us 
to anything open to serious doubt. 
In studying erupted rocks of different characters, we see that at one extreme they 
are as truly igneous as any furnace-product, and, at the other extreme, hardly, if at 
all, distinguishable from certain deposits met with in mineral veins, which furnish 
abundant evidence of the preponderating, if not exclusive, influence of water, and 
have very little or nothing in common with products certainly known to have been 
formed by the action of heat, and of heat alone. Between these extremes there is 
every connecting link, and in certain cases it is almost, if not quite impossible to say 
whether the characteristic structure is due more to the action of heat than of water. 
The great question is, whether the presence of a small quantity of water in the 
liquid or gaseous state is the true cause of very well-marked differences in structure ; 
