136 Reports & Proceedings — Geoloylcal Society of London. 



westwards, and cuts right through the limestone in a deep canon - 

 the latter breaches the northern fault-scarp where the downthrow- 

 is greatest, and from here the bed of the stream lies in the gypsum 

 of the valley bottom, except for a short stretch between two faults,, 

 where the limestone floor is again exposed to view. The stream 

 finally breaks through the southern limestone wall, and so makes 

 its escape from the valley. 



These conditions imply that the river assumed approximately 

 its present direction when there was a cover of gypsiferous beds over 

 the whole area, and that, although younger than the initiation 

 of the faulting, it is older than the sculpturing of the inside of the 

 fault-trough. The eastern end of the trough is again tapped by 

 another and smaller stream with a precisely similar development. 



It might be surmised from the excellent preservation of the 

 fluted surfaces on the fault-scarps that the last movement took 

 place at a very recent date, but the behaviour of the river in crossing 

 the scarps does not favour this supposition. The succession of 

 events appears to have been as follows : — 



(1) Formation of the trough with gyjDsum-infilling by partial collapse of 



the anticline. 



(2) Levelling of the gypsum surface and develoj)ment of a stream across 



the position, of the buried trough. 



(3) The stream, cutting down through gypsum, discovers the faults, 



etches them out, develops subsequents along them, and thus 

 gradually eats out the gyjisum filling the rift, until the present 

 topography results. 



January 7, 1920.— Mr. G. W. Lamplugh, F.R.S., President, in 



the chair. 



1. " On Syringothyris Winch ell and certain Carboniferous 

 Brachiopoda referred to Spiriferina D'Orbigny." By Frederick 

 John North, B.Sc, F.G.S. 



This paper is the outcome of a suggestion made in 1913 by 

 Professor T. F. Sibly, who pointed out the desirability of an attempt 

 to remove the uncertainty which had hitherto existed in the naming 

 of the British species of Syringothyris, and of the Carboniferous 

 Sj)iriferids possessing a lamellose surface ornament, which it was 

 customary to refer to Spiriferina because there was no other genus 

 for their reception, although it had long been recognized that few, 

 if any of them, really belonged to that genus. 



After indicating the exact sense in which certain frequently 

 occurring terms are used, and reviewing the history of previous 

 research, the author discusses the history in Avonian times of the 

 genus Syringothyris, and suggests a classification of its species. 



Variations , due to time, to environmental conditions, and to 

 distribution in space, ai^e recognized, and distinctive names are given 

 to the mutations characteristic of certain horizons. 



The syrinx (it is suggested) was a special arrangement called 

 into existence to control the direction of and to support the 



