39^ 



NA TV RE 



\_Auo-!isi 26, 1880 



but the more completely crystalline volcanic 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 crystallised in feathery 

 slceletons. 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 

 himilar rocks artificially melted and slowly cooled. The large 

 solid crystals in specimens from other localities sometimes show 

 that towards the end of their gi-owth small flat prisms have 

 developed on their surface, analogous to those first deposited in 

 the case of the artificial products. 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 diiferent 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 tu o classes are only just distinctly connected 

 by certain exceptional crystals and by structural characters 

 w hich, 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, whicli 

 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 cafes. In the arti- 

 ficial products it is in the direction of complex skeletons, which 

 are not seen in the natural rock, but in the natural rock it is in 

 the direction of large simple solid crystals, which are not met 

 with in the artificial products. Tliere is a far closer analogy 

 in the case of partially vitreous rocks, which, independent of the 

 true glassy base common to them and the artificial products, 

 often contain analogous crystalline needles. Even then, how- 

 ever, we see that in the artificial products the crystals tend to 

 develop into complex skeletons, but in the natm-al 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 tnily fused. The simple, large, and charac- 

 teristic crystals of such minerals as augite, felspar, leucite, and 

 olivine often contain so many thoroughly well-marked glass in- 

 closures 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 tlie subsequent action of water. I con- 

 tend that these glass inclosures prove that many of our Britisli 

 erupted rocks were of as truly igneous origin as any lava flowing 

 from a modern volcano. The difference between tlie 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 fir.^t 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 sat sfactory 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 beeen roughly .sketched in out- 

 line. I cannot but think that this mu,t 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 speci- 

 mens the effects of much more rapid cooling are perfectly well 

 rnarked. The base of the rock when in close contact is some- 

 times 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 character- 

 istic of slags, or at least no such passage as I should have ex- 



pected 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 w hat difference 

 there is in this latter is in the direction of the structure charac- 

 teristic 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 ccmsolidation of the rock, we can scarcely hesitate to con- 

 clude that it must have had some active influence ; but in the 

 ca-c 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, -vihether this water may not have 

 considerable influence on the gron-th 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 experi- 

 ment. M. Daubree infoi-ms 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 formation of erupted lavas to justify 

 our looking upon the explanation I have suggested as anything 

 more than sufliciently plausible, in the absence of more complete 

 experimental proofs. 



Granitic 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 exclu- 

 sively of solid crystals, analogous in character only to slag- 

 crystals of veiy 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 charac- 

 teristic 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 bmshes. There are no solid crystals of felspar, 

 hornblende, and quartz, of which the natural rock is mainly com- 

 posed, 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 natur.al 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 modi- 

 fied 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 cavitie.s, 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 ; 

 n hereas 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. 



Confining our attention, then, to extreme cases, we thus see 

 that rocks of the granitic type differ in a most characteristic 

 manner from (he products of artificial igneous fusion, both in the 

 structure of the crystals and in containing liquid water inclosed 

 at the time of their formation. The question then arises 



