part 2] THE AVOIS'IAIS^ OF EKOADFIELD DOWN. 227 



o. The Caeboj^iferous Limestone (Ayoxiais^) of Bkoadfieli> 

 Down (Someeset). By Fredeeick Steettois^ Wallis^ 

 M.Sc, F.a.S. (Read Januaiy 18th, 1922.) 



[Abstract.] 



Thk greatl}^ denuded periclinal uplift of Broadfield Down is 

 mapped and described in terms of Vauglian's system of zonal 

 classification of the Avonian. All zones, with the exception of K, 

 are present, and subzones Z^ and D^, hitherto unrecorded from this- 

 area, are proved to occur. 



A well-marked faunal assemblage (' Fossiliferous Level'), of nO' 

 great vertical extent, is described from the top of S^, and is shown 

 to constitute in this area a datum-line, useful in the field, for the 

 determination of the junction between the S^ and S.^ subzones. 



Pustula elegans (M'Coy) is here for the first time recorded 

 from the S^ subzone in the eastern part of the South- Westeni 

 Province. 



Both lithologically and palseontologically the area liolds an inter- 

 mediate position, and forms a link between the developments of 

 the Bristol and the Mendip areas. Thus C^ of Broadfield Down 

 is composed of fossiliferous, massive, grey limestones similar to- 

 that of C, of the MendijD area, while the lower part of C^ is directly 

 comparable Avith the Laminosa Dolomites of the Bristol area. 



The non-occurrence of igneous rocks at the head of G-oblin 

 Coombe, where normally they would be expected to occur, is 

 explained by a system of thrust-faulting from the South. 



Discussion. 



Prof. S. H. Reynolds referred to the fact that, almost wherever 

 detailed mapping of the Carboniferous Limestone of the Bristol 

 district was undertaken, evidence of reversed faulting of pre-Triassic 

 date was obtained. By considering certain other faults of G-oblia 

 Combe to be reversed, the Author had provided an adequate explana- 

 tion of the non-occurrence of the igneous horizon at certain spots- 

 where evidence of its presence would have been expected. 



Mr. E. E. L. Dixon remarked that, thanks to the inspiring, 

 work of Yaughan and the energy of Prof. Reynolds, the Bristol 

 school of geology, and other workers, a clear insight into the geo- 

 graphy of Lower Carboniferous time was being rapidly gained. 

 He was glad to see this important link between Bristol on the 

 north and the Mendips on the south so thoroughly described. 

 After asking for details of the lithological characters of Horizon y,. 

 he mentioned that the change in the facies of C^ and C^ as one 

 goes south from Bristol could be paralleled in Gower and Pem- 

 brokeshire. The thickening and also the ultimate disappearance 

 of the Caninict Oolite were results of the southward deepening of 



