256 Dr. A. E. Trueman — 



the predominantly sandy character of the deposit. It would be 

 difficult to find better evidence of the conformable succession of 

 volcanic rocks on sedimentary than that offered by the Elsburg- 

 Ventersdorp passage beds in that j)art of Heidelberg so far surveyed 

 in detail." 



In this connexion I would point out that coarse boulder beds 

 forming the upper part of the Ventersdorp System and containing 

 large pieces of auriferous conglomerate and quartzite evidently 

 derived from the destruction of Witwatersrand beds have been 

 recorded as resting on Lower Witwatersrand beds at Ratzekaiskraal 

 and Makokskraal to the north-west of Ventersdorp.^ Molengcaaf 

 also, in describing similar boulder beds at Zendelingsfontein to the 

 west of Klerksdorp and at Kroomdraai to the north of Krugersdarp, 

 lefers to their unconformable j)osition on Witwatersrand beds.^ 



This evidence of the existence of a break somewhere in the 

 succession of the Ventersdorp system, taken in conjunction with 

 Dr. Rogers' conclusions as to the conformable relation of the volcanic 

 .beds, forming the base of the Ventersdorp system, to the Upper 

 Witwatersrand makes it appear probable that further work may 

 rishow the necessity of placing the division between the two systems 

 jSbt a different horizon. 



The Use of Gryphaea in the Correlation of the 

 Lower Lias. 



By A. E. Trueman, D.Sc, F.G.S., University College of Swansea. 



TN the course of recent investigations in the littoral Lias of South 

 -*- Wales and parts of Somerset the writer has frequently found 

 that almost the only fossils in some of these deposits are species 

 of Ostrcea and Gryphcea, which occur in regular sequence and are 

 of considerable value in correlation. On consulting the literature 

 describing these genera it became apparent that the specific names 

 have been frequently misapplied. From correspondence with other 

 students of Liassic rocks it also appeared that there is some 

 difference of opinion concerning the precise horizons at which such 

 species are to be found. 



It was therefore necessary to investigate the various forms of 

 Ostrcea and GryphcBa found in the lowest zones of the Lower Lias, 

 and also to examine sections of the zones where they are norma.lly 

 developed in order to correlate the ammonite sequence with the 

 Ostrcea-Gryphcea sequence. This paper contains a brief account 

 of the results of these inquiries. 



^ Hatch, " The Boulder Beds of Ventersdorp, Transvaal " : Trans. Geol. 

 Soc. S.A., vol. vi, 1903, p. 95. 



2 Molengraaf, " A hitherto unrecognized Formation underlying the Black 

 Beef series" : Trans. Geol. Soc. S.A., vol. vi, 1903, p. 68. 



