TRANSACTIONS OF SECTION C. ' 565 



Section C— GEOLOGY. 

 President of the Section— H. Clifton Sobby, LL.D., F.R.S., F.G.S. 



THURSDAY, AUGUST 2Q. 



The President delivered the following Address : — 



In selecting a subject for an address to be given in accordance with the custom of 

 my predecessors, I was anxious that it should he, iu some way or other, connected 

 with the locality in which we have met. If I had been adequately acquainted with 

 the district, I should have thought it incumbent on me to give such an outUne of 

 the general geology of the surrounding country 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 appeared 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 \iew. This subject is the comparative structure of artifi- 

 cial 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 pecu- 

 liarities 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 of rocks. It would, however, be impossible in the time 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 attention, and yet, as I think, would be ver}- 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 somewhat obscure, 

 owing to the want of adequate experimental data or sufficient knowledge of general 

 physical laws. 



A considerable amount of attention has already been paid to the mineral con- 

 stitution of slags, and to such peculiarities of structure as can be learned indepen- 

 dently of thin microscopical sections. A very complete and instructive work, 

 specially devoted to the subject, was published by Von Leonhard about twenty-two 

 years ago, just at the time when the microscope was first efficientlj' applied to the 

 study of rocks. Since then Vogelsang and others have described the micro- 

 scopical structure of some slags in connexion with their studies of obsidian and 

 other allied volcanic rocks. At the date of the publication of Von Leouhard's 

 work the questions in discussion diflered materially 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 most 

 unequivocal evidence ; and I am not at all surprised at this, since, as I shall show, 

 there is such a very great difference in their characteristic structure 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 



