152 PROF. o. T. joxES OX [vol. Ixxvii, 



peculiar rocks continuously from Tarannon to Conway in North 

 Wales, where again tliey underlie the Wenlock, and naturally 

 •concluded that they were of the same age in these two localities. 

 On Tarannon the shales are underlain by a great grit group ; hut, 

 as these beds disappear before reaching Conwa^'^, Aveline concluded 

 that they had been overstepped by the Tarannon, and that in con- 

 sequence the two groups were not conformable one to the other. 

 He had previously found that the pale shales behaved in a similar 

 way towards the Lower L Ian do ver}' rocks of the Llandovery district; 

 and as the grits of Tarannon showed some similarity of lithological 

 •characters to supposed Lower Llandovery grits of various parts of 

 Wales, they were assigned on these grounds to that formation. 



When, in 1870, Lapworth discovei-ed in the Tarannon Shales of 

 •Conway many of the same species of graptolites as those that he 

 liad found in the lower part of the Gala Group, he concluded that 

 the Tarannon Shale occupied the S3\steniatic position of that group. 

 This correlation of the Tarannon with the Gala, and the Birkhill 

 with the Lower Llandovery, left no room for the Upper Llandovery; 

 whereas, in the Llandovery district that formation lies between the 

 Tarannon and the Lower Llandovery. This conflictino- evidence in 

 regard to the relations of these foimations received no explanation 

 until the Tarannon area was carefully revised by Dame Ethel Shake- 

 spear (Wood). ^ The peculiar jDurple and green shales (renamed the 

 DolgauBeds) to which the name Tarannon Shales was given by 

 Aveline are underlain conformably by a great series of rocks which 

 <3ontain many species of graptolites. Those of the shales and of 

 the Talerddig Grits, on which they rest, are closely allied. Below 

 the grits are the Gelli and Brynmair Groups, and below the latter, 

 representatives of the Upper Birkhill and of the Lower Birkhill 

 faunas were proved. The Dolgau Beds cannot be dissociated from 

 the underlying strata, and on faunal grounds all four groups make 

 lip one homogeneous formation. 



Dame Ethel Shakespear considered that the Tarannon Shales 

 ■of Conway are equivalent to the two lower groups only of the 

 Tarannon sequence, and the two upper groups are not represented ; 

 but, in a later account of the district, Miss G. L. Elles ~ records 

 "the highest zone (Jlonoc/rajyhis crenulcdus^ in beds which pass 

 up conformably into the Wenlock. The underlying zone of 

 M. griestonensis is believed to be represented by unfossiliferous 

 ;shales. Detailed comparison of the faunas proved the exact 

 parallelism of the Tarannon Series with the Gala Beds of the 

 •South of Scotland : the true Tarannon Shales being represented in 

 •all probability hy the Hawick rocks, which occupy a position 

 immediately beloAV the Biccarton rocks or the equiA'^alents of the 

 Wenlock Shale. 



The investigation of the Tarannon and Conwaj^ areas thus 

 proved that the ' Pale Slates ' mapped b}^ Aveline are not a homo- 

 .geneous formation, but a group in which, while certain lithological 

 characters are retained, the lower limit varies from point to point 

 •of the outcrop. 



1 Q. J. G. S. vol. Ixii (1906; p. 644. ^ jj,^-^. yoj, i^v (1909) pp. 186 & 189. 



