698 MISS E. M. B. AVOOD ON THE [Nov. I906, 



(5) Lithological. 



We have seen that, even within the limits of the Tarannon 

 district, the sediments tend to become more argillaceous, and con- 

 sequently thinner, as the}' are followed from the south-east to the 

 north-west ; and if we leave the Tarannon area and go some 

 12 miles farther north, to the neighbourhood of Llan-y-Mawddwy, 

 this change becomes still more marked. In the summer of 1905 

 Miss Elles and I made a traverse of the strata in this district 

 from rocks with Bala fossils, as exposed in the upper waters of 

 the Afon Dyfi, below Tr Eryr, up to Wenlock Beds at the summit 

 of Carreg-y-big. In the intervening ground, along the course 

 of the Cwm Cerddyn, the Gelli, Talerddig, and Dolgau Groups of 

 the Tarannon Series were found to be present with their charac- 

 teristic graptolites. The grit-beds of the Talerddig Group, however, 

 are here practically confined to one horizon near the top ; and the 

 group, as a whole, is more shaly and thinner than in the Tarannon 

 district. The purple bands in the Dolgau Group above (local 

 i Tarannon Shales ' of the Survey) have here completely disappeared, 

 but the graptolitic black shale-bands at the base are well-marked. 



Proceeding now to the extreme northern limit of the Welsh 

 * Tarannon ' belt as mapped at Conway, we find that the collective 

 amount of sediment present has diminished to a still greater extent. 

 It is true that at Conway the upper division of our Tarannon Series 

 has not been recognized, if indeed it is not overlapped by the Wen- 

 lock Shale ; but it may be expected that the whole of the Tarannon 

 is here dwindling to the restricted development which it has in the 

 Lake District beyond (Browgill Beds), where its collective thickness 

 is only some 200 feet. 



Still farther north, however, in the South of Scotland, we find a 

 close parallel to the Tarannon Series as developed in the Tarannon 

 district ; for in Girvan the beds referred to the Tarann on-Gala Series 

 are nearly 2000 feet thick, while they are perhaps twice as thick in 

 the central, or Gala and Queensberry parts of the Scottish Uplands. 



Thus, the results arrived at in the Tarannon district enable us 

 not only to bring the Welsh development of the Tarannon into 

 line with that of other districts, but also more or less to reconcile 

 the various views which are found in geological literature with 

 respect to the Tarannon as a whole. The Tarannon Series, 

 as here defined, includes all the palaeontological zones 

 which have been hitherto assigned to it, and it fills 

 up the whole of the period of time intervening be- 

 tween the Llandovery below and the Wenlock above. 

 It includes the oldest and youngest beds which have been mapped 

 as Tarannon by the Survey in Wales ; and, in the Tarannon district, 

 at all events, the thickness of the Series is practically equivalent 

 to its maximum development elsewhere. 



In conclusion, I wish to express my thanks to the Council of 

 Birmingham University for a research-scholarship in aid of my 

 work ; to Miss Elles, Miss C. Chamberlain, Miss H. Clark, Mr. A. R. 



