SfiG KEPORT— 1889. 



5. The Physiography of the Lower Ttias} 

 By T. Mellard Reade, C.E., F.G.S. 



After an introduction describing the various views held by geologists as to the 

 origin of the Triassic roclis of Britain, the author stated that circumstances have 

 lately provided an opportunity for a detailed examination by him of considerable 

 areas of the Trias in the North- West of England. As a result of this he has been 

 drawn towards a consideration of the attractive speculations on the origin of these 

 rocks which have emanated from various geologists. 



Describing the lacustrine theory suggested by the late Mr. Godwin- Austen and 

 supported by Sir Andrew Ramsay, the author agrees with Professor Bonney that 

 it does not account satisfactorily for the extensive areas of marine current-bedded 

 sandstones constituting the base of the Trias, and known as the Bunter. 



He considers that the prevalence of pebble-beds in these rocks, as well as the 

 absence of thick beds of marl, is inconsistent with such an origin. Further, re- 

 ferring to the sub-aerial river delta theory brought forward in substitution by 

 Professor Bonney, he contends that, while explaining the deposition of current- 

 bedded sandstones, there is no evidence of the existence at the time of such peculiar 

 physiographical conditions as would be required for the building up of a sub-aerial 

 delta of sand from 1,200 to 2,000 feet thick. 



Such a delta would require to be fed from a high plateau or from Alpine granitic 

 ranges, of the former existence of which we have no evidence. The author considers, 

 from his studies of mountain ranges, that such a permanent feature of the earth's 

 surface, had it existed, would not have so totally disappeared, while lesser ranges 

 like the Pennine Chain have survived. 



It is also pointed out that the rivers of Asia and Africa feeding deltaic sands, 

 which the Bunter is compared to, frequently disappear in them by absorption and 

 desiccation leaving saline deposits, while in the Bunter of the North- West of 

 England there is little or no evidence of such conditions having obtained. 



In suggesting another possible origin for the Bunter sandstones, it is pointed 

 out that the Triassic rocks which are preserved appear to lie in the deeper part of 

 the basins in which they were deposited, and that there has been here and there a 

 remarkable preservation of the ancient orographic features, modified, it is true, by 

 folding and faulting. In support of this it is shown that a subsidence of 400 feet 

 would cover most of the Trias by the sea, which would approximately follow in its 

 margin the present Triassic boundaries. 



Where the Triassic deposits are bounded by the mountains of Wales and by the 

 Pennine Chain or other high land, the overlapping of the present boundaries by the 

 sandstones was limited; but there is every reason to suppose that there was a wide 

 extension of them occupying the site of the Irish Sea. It is contended that these 

 features most probably represent subsided valleys or arms of the sea previously 

 occupied by partly denuded Permiaus ; and the author thinks that tidal action is 

 capable of selecting the materials for and constructing, under conditions pointed 

 out, thick massive beds of sandstone, current-bedded, with intercalated conglo- 

 merate beds or with well-rounded quartzite pebbles distributed sparsely through 

 the rock or in nests, as found in all these forms in the Bunter sandstones. Two 

 papers by the author on Tidal Action as a Geological Agent in the ' Philosophical 

 Magazine,' and the ' Proceedings of the Liverpool Geological Society ' were referred 

 to in explanation of the physical questions involved. 



It is suggested that a granitic area, such as would be exposed now by an eleva- 

 tion of, say, 1,000 feet, occupying the site of the English Channel, together with 

 the Old Red Sandstone beds of the anticlinal axis connecting the Mendips with 

 the Belgian coal field, reinforced by the sand derived from the immense destruction 

 of carboniferous sandstones which, we know, has taken place in the Pennine 

 Chain and in other areas, together with the denudation of the Old Red of Here- 

 ford, may have supplied the materials for the Triassic rocks, which were distributed 

 and built up by tidal action in straits, seas, and bays. The details of the process 



' Publiahed in extenso in the Gecl. Mog. for December 1889, pp. 540-558. 



