TEANSACTIONS OF SECTION C. 569 



very characteristically xinlike tbat of the artificial products. I have most care- 

 fully examined all my sections of modern and ancient volcanic rocks, but cannot 

 find any in which the augite or magnetite is crystallised 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 locks 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 ex- 

 clusively of what I believe is Humboldtilite, the crystals are indeed uniformly as 

 simple and solid as those in natural rocks, but the examination of difl'erent blow- 

 pipe 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 b}' structural cha- 

 racters 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 difl'ereut 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 in 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, independent of the true glassy base common to 

 them and the artificial products, often contain analogous crystalline needles. Even 

 ■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 en- 

 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 cn'Stalline, 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 fiowing 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 explanation 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 in outhne. 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 perfectly well 

 marked. The base of the rock when in close contact is sometimes so extremely 

 fine-grained tbat 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 



