570 EEPORT— 1880. 



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, hut one of my speci- 

 mens 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 con- 

 clude 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. Daubree 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 formation of 

 erupted lavas to justify om- looking upon the explanation I have suggested as 

 anything more than sufficiently plausible, in the absence of more complete ex- 

 perimental proofs. 



Granitic Mocks. 



I now proceed to consider rocks of another extreme type, which for distinction 

 we may call the gi-anitic. On the whole, thej- 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 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 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 natiu-al 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 



