216 Dr. C. Callaway — On Plagioclinal Mountains. 



grey or nearly white, occasionally dark grey or purple, from organic 

 matter or the material, probably Silurian slates, from which they 

 were derived. Although they contain a few nodules of siliceous clay 

 ironstone in some places — very sparingly — and not unlike those of 

 the Coal Measures, the small quantity of iron the clays contain will 

 be inferred when I mention that all qualities are largely used for the 

 manufacture of tiles and coarse pottery ; for which, of course, ferru- 

 ginous clays would be utterly useless. 1 Yet of these clays Mr. 

 Swanston pens the following remarkable passage, on the assumption 

 that they are of the same age and origin as the Antrim h-on-ores : — 



" Should subsequent researches prove these to be the same, it may 

 safely be inferred that one reason why the Lough Neagh beds still 

 retain their clayey character is owing to the simple fact that the 

 later outflows of Miocene basalt did not reach them, and convert 

 them into iron-ores, etc., similar to those so largely developed to the 

 east and north of Antrim" (!). The simpler fact that they are non- 

 ferruginous clays is the easier mode of explanation. Ex nihilo nihil jit. 



If the Lough Neagh clays were of intra-Miocene age, and formed 

 in the same way and time as the Antrim ore deposits, they would be 

 found resting only on basalt, or absent where basalt does not occur. 

 They however overlap the previously denuded basalt, and their 

 continuation is found in several places resting on Triassic rocks. 



Mr. Swanston is doubtless unaware of these facts, which are, I need 

 not observe, completely antagonistic to his doctrine as to the age of the 

 clays. And but that I fear to trespass on the kindness of the Editor 

 of the Geological Magazine, whose space is valuable, it would be 

 easy to give numberless' other facts as overwhelming proofs of the 

 post-Miocene age of the Lough Neagh clays, and of the lake itself. 



Returning to the Crumlin shells. I must now, at the cost of 

 repetition, point out that even if they are hereafter proved to be 

 Glacial fossils, such a matter can hardly be evidence in favour of the 

 intra-Miocene age of the lake : yet this is apparently the conclusion 

 to which Mr. Swanston's arguments would lead us. 



With regard to the silicified wood, I can only say that it has never 

 been found in any of the numerous clay-pits to the south of the 

 lake. And in the only case where Barton records its existence 

 in situ, it may have belonged to the drift. Excavations and borings 

 in the same locality in the true lacustrine clays revealed none of it ; 

 and the origin of this wood is a matter still enveloped in great 

 obscurity. 



YIII. — On Plagioclinal Mountains. 

 By Charles Callaway, M.A., D.Sc. (Lond.), F.G.S. 

 niHE object of this paper is to show that the received explanations 

 J_ of the structure of mountain chains will not account for the 

 formation of certain Precambrian ranges, and to state facts in sup- 

 port of a new theory of the origin of some mountains. A brief 

 summary of the ordinary views of the subject will first be necessary. 



1 The analysis of these clays, given by Sir Robert Kane, shows them to be 

 remarkably free from iron. 



