390 



NA TURE 



^^Aiigust 26, 1880 



some of ihem were again lifted up several thousands of feet after 

 the close of the Miocene epoch. 



The deposition of salts from aqueous solutions in inland lahes 

 and lagoons appears to have taken place through all time — 

 through Silurian, Devonian, Carboniferons, Permian, Triassic, 

 Jurassic, Cretaceous, Eocene, Miocene, and Pliocene epochs — 

 and it is going on now. 



In like manner fresh- water and estuarine conditions are found 

 now in one region, now in another, throughout all the forma- 

 tions or groups of formations, posnbly from Silurian times 

 onward ; and glacial phenomena, so far from being confined to 

 what was and is generally still termed the Glacial epoch, are 

 now boldly declared, by independent witnesses of known high 

 reputation, to begin with the Cambrian epoch, and to have 

 occurred somewhere, at intervals, in various formations, from 

 almost the earliest Palxozoic times down to our last post-Pliocene 

 " Glacial epoch." 



If the nebular hypothesis of astronomers be true (and I know 

 of no reason why it should he doubted), the earth was at one 

 time in a purely gaseous state, and afterwards in a fluid condi- 

 tion, attended by intense heat. By and by consolidation, due to 

 partial cooling, took place on the surface, and as radiation of 

 heat went on, the outer shell thickened. Radiation still 

 going on, the interior fluid matter decreased in b'.:lk, and, by 

 force of gravitation, the outer shell being drawn towards the 

 interior, gave way, and, in parts, got crinkled up, and thi-, 

 according to cosmogonists, was the origin of the earliest moun- 

 tain-chains. I make no objection to the hypothesis, which, to 

 say the least, seems to be the best that can be offered, and looks 

 highly probable. But, assuming that it is true, these hypo- 

 thetical events took place so long before authentic geological 

 history began, as written in the rocks, that the earliest of the 

 physical events to which I have drawn your attention in this 

 address was, to all human apprehension of time, so enormously 

 removed from these early assumed comical phenomena, tliat 

 they appear to me to have been op eoinparativcly quite modern 

 occurrettee, and to indicate that,f>oni the I.aurentiati epoch dotvn 

 to the present Jay, all the physical clients in the history of the earth 

 have varied neither in kijid ttor in intensity from those of which 

 we now have experience. Perhaps many of our British geologists 

 hold similar opinions, but, if it be so, it may not be altogether 

 useless to have considered the various subjects separately on 

 which I depend to prove the point I had in view. 



SECTION C 



OrENiNG Address by H. C. Sorev, LL.D., F.R.S., &c., 

 President of the Section 

 In selecting a subject for an address to be given in accord- 

 ance with the custom of my predecessors, I was anxious that 

 it should be in some way or other connected with the locality 

 in which we have met. If I Iiad been adequately acquainted 

 with the district, I should have thought it incumbent on me to 

 give such an outline of the general geology of the surrounding 

 countiy as would have been useful to those attending this 

 meeting. I am, however, practically a stranger to South Wales, 

 and must therefore leave that task to others. On reflecting on 

 the various subjects to which I might have called your attention, 

 it appears to me that I could select one which would be 

 eminently appropriate in a town and district where iron and 

 copper are smelted on so large a scale, and, as I think, also 

 equally appropriate from a geological point of view. This 

 subject is the comparative structure of artificial slags 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 oj 

 rocks. It would, however, be impossible in the lime 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 propose 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 at'.ention, and yet, as I 

 think, would be vei-y instructive if we could fully understand 

 their meaning. Here, however, as in so many cases, the 

 observed facts are clear enough, but their full significance is 

 somewhat obscure, owing to the want of adequate experimental 

 data, or of sufficient knowledge of general physical laws. 



A considerable amount of attention has already been paid to 

 the m.ineral constitution 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 

 tH enty-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 connection with their study of obsidian and 

 other allied volcanic rocks. At the date of the publication of 

 von Leonhard's work the questions in discussion differed mate- 

 rially 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 mo^t unequivocal evidence. I am not at all surprised at this, 

 since, as I shall show, there is such a very great difference in 

 their characteristic struc'ure and that of the artificial products of 

 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-cavitie~," there would often be no positive 

 ]iroof of their igneous origin. There was also considerable 

 doubt as to the manner in which certain minerals in volcanic 

 racks had been generated. The observed facts w ere 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 

 wiiether in particular cases certain minerals had been formed 

 exclusively by one or other proces=, or sometimes by one and 

 sometimes by the other, or by the combined action of water and 

 a 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, however, applies more to the origin 

 of minerals occurring in cavities than to those constituting a 

 part cf true rock masses, to which latter I shall almost exclu- 

 sively 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, bat, in 

 other ca'es 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 extremity hardly, if at all, dis- 

 tinguishable 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 farmed by the action of heat, and of heat alone. 

 Between these extremes there is every connecting link, and irt 

 certain cases it is almost, if not quite, impossible to say whether 

 the characteristic strncture 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 vei7 well-marked differences in structure, or whether gi-eater 

 pressure and the necessarily slower rate of coohng were not the 

 more active causes, and the presence of water in one state or 

 another was merely the result of the same cause. This is a 

 ijuestion which ought to be 'solved by experiment, but I fear it 

 would be almost impossible to perform the necessary operations, 

 in a satisfactory manner. 



What I now propose to do is to describe a particular cla=s of 

 facts which have lately attracted my attention, and to show that 

 the crystalline minerals in products known to have been formed 

 by the action of heat alone have a certain very well-mdrked and 

 character!; tic structure, whi;h is gradually modified as we pass 



