ON THE TRIASSIC STRATA OF THE SOUTH-WESTERN COUNTIES. 459 



30. On the Chronological Value of the Triassic Strata of the 



South-western Counties. By W. A. E. TJssher, Esq., E.G.S*, 



of H.M. Geological Survey. (Read March 20, 1878.) 



[Communicated by permission of the Director-General of the Geological 

 Survey of the United Kingdom.] 



In no part of the south-western counties is a more detailed section 

 of the Triassic rocks exhibited than on the South-Devon coast. It 

 is the district in which the subdivisions seem to have attained their 

 maximum thickness ; and though they may be traced in unbroken 

 conformity to Williton, near the shores of the Bristol Channel, the 

 absence of the lower members of the group to the east of Taunton, 

 and a study of the variable relations of land and water during the 

 period in different parts of the area, render the proof of the follow- 

 ing propositions a necessary supplement or second part of the paper 

 which I have already had the honour of communicating to the 

 Society *. 



Eirst proposition. — That the South-Devon and West-Somerset 

 Triassic area was not connected with that of Gloucester and the 

 midland counties till the later stages of the Keuper period. 



Proof. — (a) The tailing-off of Triassic sediments towards the 

 Mendip country. 



(b) The representation of the Trias on the north of the Poldens 

 by marls alone, the Dolomitic conglomerate being a contemporary 

 beach. 



(c) The abnormal thinning-out of the Trias on the north of the 

 Mendips. Thus at Tyning pit, near Radstockf, it was bottomed at 

 186 feet ; at Norton-Hill pit at 172 feet ; at Batheaston (under 

 Rhsetic) at 54 feet ; in Mangotsfield cutting at 25 feet. The general 

 results arrived at by Mr. Moore give an average thickness of 50 

 feet to the Trias within the coal-basin. 



(d) The fact that these marls, being in every case the directly 

 antecedent deposits to the Rhsetic beds, must be the equivalents of 

 the Upper Keuper Marls of the midland counties, whilst their at- 

 tenuation in the intervening districts proves the non-existence of 

 connexion between the Triassic areas of the midland counties and 

 those of West Somerset and Devon till a late stage in the deposition 

 of the Upper Keuper Marls. 



Second proposition. — That the area east of Taunton and south of 

 the Quantocks was not submerged before the deposition of the Lower 

 Keuper Sandstone and probably not till the later stages of its for- 

 mation, the Quantocks acting as a barrier between the earlier 

 deposits in the Bridgewater area and those of the Watchet valley. 



Proof. — (a) The Trias of Taunton Yale is composed of Upper 

 Keuper Marl, beneath which sandstones are visible on the south 

 flanks of the Quantocks in places. 



* Quart. Journ. Geol. Soc. vol. xxxii. p. 367. 

 t Ibid. vol. xxiii. p. 458. 



