472 Notices of Memoirs—Dr. H. C. Sorby’s Address. 
melted and slowly cooled basalt is entirely crystalline, and has a mineral composition 
closely like that of the natural rock, its mechanical structure is very different, being 
identical with that of blowpipe beads and slags. : 
Volcanic Rocks.—Passing now to true natural igneous rocks, we find some, like 
obsidian, which closely correspond with blowpipe beads, slags, and artificially melted 
rocks, in having a glassy base, through which small crystalline needles are scattered ; 
but the more completely crystalline voleanic rocks have, on the whole, a structure 
very characteristically unlike that of the artificial products. I have most carefully 
examined all my sections of modern and ancient volcanic rocks, but cannot find any 
in which the augite or magnetite is crystallized in feathery skeletons. In the case 
of only one single natural rock, from a dyke near Beaumaris, have I found the 
triclinic felspar arranged in just the same fan-shaped, brush-like groups as those 
in similar rocks artificially melted and slowly cooled. The large solid crystals in 
specimens from other localities sometimes show that towards the end of their growth 
small flat prisms were developed on their surface, analogous to those first deposited 
in the case of the artificial product. In slags composed almost exclusively of what 
I believe is Humboldtilite, the crystals are indeed uniformly as simple and solid as 
those in natural rocks, but the examination of different blowpipe beads shows that 
no fair comparison can be made between altogether different substances. We must 
compare together the minerals common to the natural and the artificial products, 
and we then see that, on the whole, the two classes are only just distinctly connected 
by certain exceptional crystals and by structural characters which, as it were, overlap 
enough to show that there is a passage from one type to the other. In the artificial 
products are a few small, solid crystals of both augite and a triclinic felspar, which 
closely correspond to the exceptionally small crystals in the natural rocks; but the 
development of the great mass of the crystals is in a different direction in the two 
cases. In the artificial products it is in the direction of complex skeletons, which are 
not seen in the natural rock; but in the natural rock it is m the direction of large 
simple solid crystals, which are not met with in the artificial products. There is a 
far closer analogy in the case of partially vitreous rocks, which, dependent of the 
true glassy base common to them and the artificial products, often contain analogous 
crystalline needles. yen then, however, we see that in the artificial product the 
crystals tend to develop into complex skeletons, but in the natural rocks into simple 
solid crystals. 
It must not be supposed that these facts in any way lead me to think that 
thoroughly crystalline modern and ancient volcanic rocks were never truly fused. 
The simple, large, and characteristic crystals of such minerals as augite, felspar, 
leucite, and olivine, often contain so many thoroughly well-marked glass enclosures, 
as to prove most conclusively that when the crystals were formed they were surrounded 
by, and deposited from,-a melted glassy base, which was caught up by them whilst it 
was still melted. This included glass has often remained unchanged, even when the 
main mass became completely crystalline, or has been greatly altered by the subsequent 
action of water. I contend that these glass enclosures prove that many of our 
British erupted rocks were of as truly igneous origin as any lava flowing from a 
modern volcano. The difference between the structure of such natural rocks and that 
of artificial slags must not, in my opinion, be attributed to the absence of true igneous 
fusion, but to some difference in the surrounding conditions, which was sufficient to 
greatly modify the final result, when the fused mass became crystalline on cooling. 
‘The observed facts are clear enough, and several plausible explanations might easily 
be suggested, but I do not feel at all convinced that any single one would be correct. 
That which first suggests itself is a much slower cooling of the natural rocks than is 
possible in the case of the artificial products; and I must confess that this explana- 
tion seems so plausible that I should not hesitate to adopt it, if certain facts could be 
accounted for in a satisfactory manner. Nothing could be more simple than to 
suppose that skeleton crystals are formed when deposition takes place in a hurried 
manner, and they so overgrow the supply that they develop themselves along certain 
lines of growth before there has been time to solidly build up what has been roughly 
sketched inoutline, I cannot but think that this must be a true and, to some extent, 
active cause, even if it be inadequate to explain all the facts. "What makes me 
hesitate to adopt it by itself is the structure of some doleritic rocks when in close 
contact with the strata amongst which they have been erupted. In all my specimens 
the effects of much more rapid cooling are pertectly well marked. The base ot the 
