﻿222 DE. A. VATJGHAN" ON THE PALJ20NTOLOGICAL [May I905, 



Visean. 



Seminula-Zone (S). 



Semif'eticulatus-suhzone (S 2 ) . 



Lithological character: — 



(1) Thick dolomites, with subsidiary shales (unfossiliferous), 



succeeded by 



(2) Massive limestones containing, at the top, two beds of 



highly-quartzose sandstone (' Firestone'). 

 Exposure : — 



East of the tunnel, the upper part of the dolomites and 

 shales is buried beneath a thick deposit of Dolomitic 

 Conglomerate. The massive limestone forms the end of 

 the cutting, and is well exposed in the large quarry which 

 immediately follows on the side of the railway-line. 



The stratigraphical relation of (1) and (2). — In the 

 paper already referred to, Prof. Lloyd Morgan expresses the opinion 

 that the continuity of the beds is broken between (1) and (2) by a 

 reversed fault. 1 



Towards the end of the cutting the lowest beds of (2) certainly 

 overlie the Dolomitic Conglomerate ; and Prof. Lloyd Morgan 

 explains this fact as due to a reversed fault by which 



' the beds of Mountain Limestone have been thrust up, along their dip- 

 faces, oyer basement-beds of the Trias.' (Op. cit. p. 6.) 



Of the correctness of this explanation I have considerable doubt, 

 for the following reasons ; — 



The lowest beds of (2) will be shown by palseontological evidence 

 to belong to that portion of the Seminula-Zone which immediately 

 succeeds the shales and dolomites in the Avon section. 



Again, by calculating, from dip and distance, the position of the 

 beds (2) at Tytherington above the base of the * Caniaia-Oolite ' 

 exposed in the quarry west of the tunnel, and by comparing it with 

 their position in the Avon section, it is seen that these beds actually 

 occur very approximately in the position which they should occupy 

 if the series were continuous. This conclusion merely proves that, 

 if there be a fault, it can have no appreciable throw ; but, seeing 

 that a fault in which the fault-plane is coincident with the bedding- 

 plane could have no throw, it does not disprove the existence of 

 such a fault as Prof. Lloyd Morgan suggests. 



At the end of the Carboniferous-Limestone section at Sodbury, 

 however, where the limestone comes into contact with Dolomitic 

 Conglomerate, there is a similar instance of limestone-beds over- 

 lying a mass of Dolomitic Conglomerate, except that here the 

 Conglomerate can be clearly seen to rest upon the limestone-floor 

 and to fill in a recess in the limestone-cliff. 



The Tytherington phenomenon seems to me to be a similar case, 

 and to be even easier of interpretation, for the shaly beds (1), just 



1 Proc. Bristol Nat. Soc. n. s. vol. vi (1888-91) p. 13. ' 



