Vol. 62.] TARANNON SEEIES OF TARANNON. 671 



in the Tarannon Valley from the Nant-ysgollon Shales to the base of 

 the Gelli Beds. As the Afon Iaen cuts across the main synclinal 

 axis of the country we pass twice over the complete sequence, while 

 some of the groups of strata are repeated oftener by smaller local 

 folds. The palaBontological characters presented by the various 

 groups in the two areas are practically identical, and the lithology 

 and thickness agree fairly closely. On the whole, however, the beds 

 in this northern area are more argillaceous than in the south. The 

 Gelli Beds here consist mainly of shales and mudstones, while the 

 grit-beds in the Talerddig Group are thinner and less numerous. 



The Dolgau Beds retain their usual characters, but there is a 

 greater preponderance of green rocks, and the purple bands, which,, 

 as we have seen, had a collective thickness of 175 feet in the 

 Tarannon section, have decreased to 70 feet at the eastern, and even 

 to 8 feet at the western end of the Afon-Iaen section. At the same 

 time, however, the green bands which are interbedded with them 

 appear to increase in corresponding proportion, so that the Dolgau 

 Group, as a whole, does not show much diminution in thickness. 

 At first sight, it appeared possible that there might be but one band 

 of pnrple mudstone, which had been repeated by sharp isoclinal 

 folding ; but mapping on the 25-inch scale proved beyond doubt 

 that there was no evidence for such intense folding, and that the 

 simplest interpretation of the facts is that given above. 



The variable character of its purple bands necessarily makes 

 the threefold division of the Dolgau Group adopted in this paper a 

 somewhat arbitrary one ; but the Middle Purple Sub-group is so 

 marked a feature over so large a part of the area that it can 

 hardly be disregarded altogether. Moreover, the base of the Purple 

 Group is on approximately the same horizon throughout the district. 



(ii) Section in the Railway-Cutting north of 

 Llawr-y-coed. (See fig. 2, p. 672.) 



In this northern area there occur a few supplementary sections, 

 which fall to be mentioned in this place. One of these is exhibited 

 in the railway-cutting, east of the bridge due north of Llawr-y-coed, 

 for a distance of about 170 yards. 



The northern bank of the cutting is here 20 to 30 feet high, 

 and presents us with a good section of the Dolgau Beds. Immediately 

 to the east of the bridge the Middle Purple division is shown, the 

 beds striking 10° west of north, and dipping at 40° to 45° west- 

 south-westward. There are three purple bands, respectively 11, 9, 

 and 15 feet thick, separated by two green bands of 21 and 15 feet 

 respectively. The lowest purple band is underlain by 36 feet of 

 green shales and mudstones with prominent flags, which pass down 

 into the basement-beds of the Dolgau Group containing the usual 

 black shale-bands. A few fragments of graptolites were got from 

 these shales, identical with those obtained from similar beds at 

 Pen-y-graig and elsewhere. 



The Lower Dolgau Beds are bent into a sharp fold, which is 



