Vol. 62.] THE TARANNON SERIES OF TARANNON. 691 



III. General Summary. 



(A) The Tarannon Sequence. 



Having completed the examination of the more important sec- 

 tions in the Tarannon District, we ma}' now summarize the results 

 achieved with regard not only to the lithological, but also to the 

 palseontological characters of the rocks, and compare them with their 

 equivalents elsewhere. 



We have seen that, in this Tarannon area, resting conformably 

 on Llandovery rocks below and passing up without a break into 

 Wenlock Beds above, occurs a series of strata some 3000 to 3500 feet 

 in thickness. This great rock-series, which we term the Tarannon 

 Series, is stratigraphically continuous from base to summit, and 

 includes the four divisions of the Brynmair, Gelli, Talerddig, and 

 Dolgau Groups, which, while the)' possess distinctive features of 

 their own, are bound together by common palseontological charac- 

 teristics. 



The Brynmair Group (zone of Monograptus turriculatus), 

 which constitutes the lowest division of the Tarannon Series, 

 consists of pale to dark-grey mudstones and shales, with only 

 occasional bands of flaggy material. In the overlying Gelli 

 Group (zone of Monograptus crispus), the sediments become some- 

 what coarser in character, but they vary in different parts of the 

 district. In the south, sandy flags and well-laminated shales occur 

 in almost equal proportions ; but in the north-west mudstones and 

 shales predominate. The Talerddig Group (zone of Mono- 

 graptus griestonensis) above is essentially a sandy one, and contains 

 numerous bands of thick grit which are generally massed together 

 at four or five distinct horizons. As with the underlying Gelli 

 Group, its sediments become finer as a whole when followed to 

 the north. The highest member of the Tarannon Series, namely the 

 Dolgau Group (zone of Monograptus crenulatus) (local Tarannon 

 Shales of the Geological Survey), consists of pale-green mudstones 

 and shales interbedded with a varying number of purple bands. 

 These purple bands are very prominent in the southern part of the 

 district, but almost disappear as they are traced north-westward, 

 where their place is taken by bands of a green tint. 



The Tarannon Series is overlain with complete conformity by the 

 Wenlock Series, which in this district consists of a lower shaly 

 division — Nant-ysgollon Group (zones of Cyrtograptus Mur- 

 chisoni and Monograptus riccartonensis) ; and an upper and coarse 

 gritty division — the Eynyddog Group, or Denbighshire Grits, 

 having a collective thickness of 1500 feet. 



The Tarannon Series passes down, also without a break, into the 

 Llandovery Series, which has at present been recognized in this 

 district only in the centre of the Twymyn Valley. The Llandovery 

 sediments are formed of finer material, and their component divisions 

 are much thinner than those of the overlying Tarannon and Wenlock 

 Series, and collectively only 300 or 400 feet thick. Three divisions 



