52 Prof. T. O. Bonney — Physiography of the L. Trias. 



II. — Me. Mellard Reade's Interpretation of the Lower Trias 



Physiography. 



By Prof. T. G. Bonney, D.Sc, LL.D., F.R.S., F.G.S. 



TF truth is elicited by the conflict of opinion, we ought to arrive 

 at a right conclusion as to the physical history of the English 

 Trias. Unfortunately, however, some of the hypotheses propounded 

 result, as it seems to me, from dwelling too much on certain minor 

 difficulties, or limiting the view to one group of facts, while others 

 not less important are excluded. From these objections Mr. Mellard 

 Reade's interesting communication, printed in the Geol. Mag. for 

 last December, does not appear to me exempt. 



Incidentally, however, he clears the ground by his distinct recog- 

 nition of the fact (often overlooked) that the Bunter and the Keuper 

 were deposited under very different circumstances, and by the state- 

 ment (grounded on personal observation) that the tripartite division 

 of the Bunter group is not so general or so definite as was asserted 

 by Professor Hull. This removes a difficulty in the hypothesis of 

 a fiuviatile origin, which I have always felt to be rather serious. 



Restricting his remarks to the Bunter, Mr. Mellard Reade admits 

 that the lake-delta hypothesis is unsatisfactory, and that the choice 

 lies between a fiuviatile or a marine origin. 



His main objections to the former of these two hypotheses, as I 

 gather from his paper, are the following : — 



(1) That he cannot understand the existing distribution of the 

 Bunter on the hypothesis of a fiuviatile origin. He adduces some 

 instances of his difficulties. Of these one only seems to me 

 important, the occurrence of Triassic Sandstones in the Vale of 

 Clwyd. Doubtless this is a difficulty, but I find it no more easy to 

 account for these on the hypothesis which he prefers ; so that I 

 remain where I was. The patch of Trias, low down in the Eden 

 Valley, does not seem so very anomalous, while I entirely fail to 

 see any improbability in regarding the Pennine Chain as an upland 

 which separated for a time the valleys of two rivers, draining the 

 mountainous region to the north. As for the general tendency of 

 the sandstones to thin off against the hills which bounded the 

 Triassic lowland, that is no more than what we should expect, and 

 agrees with the demeanour of the sub- Alpine drifts at the present 

 day. 



(2) That there is no instance on record "of the finding in Triassic 

 Sandstones of anything like a river channel." I should be rather 

 surprised if one had been detected. Rivers discharging sand and 

 gravel on an open lowland, and liable to frequent floods, are constantly 

 changing their courses, and the deposits are so variable that definite 

 channels are not indicated. The same objection would apply to the 

 great masses of post- Pliocene drift which border the Alps, and to 

 the nagelflue and molasse of Switzerland. 



(3) That " it is difficult to conceive of a river .... bringing 

 down nothing but sand." The sub- Alpine drifts, j-ust named, closely 

 resemble the Bunter pebble- beds, and as regards thickness are not 



