700 THE TARANNON SERIES OF TARANNON. [NOV. I906, 



struck by the remarkable agreement of the results in the Taraunon district 

 with those in the country lying to the south on the southern fringe of the 

 Mid- Wales complex. 



' One of the most interesting points, to my mind, is the restriction of the 

 typical brilliant purple mudstones to the upper portion of the Tarannon 

 Shales. In the Rhayader district these bright-coloured rocks are absent. To 

 the east, however, about 12 miles distant from the town of Rhayader, these 

 beds have been mapped by the Geological Survey and traced across country 

 towards the north, where they eventually join the same rocks in the Tarannon 

 district. I visited this eastern district myself about seven years ago, and noted 

 the typical zone-fossil Monograptus, crenulatus in the purple beds, shown on 

 the Geological-Survey maps. I was never able to understand how this bright- 

 coloured group could cross (apparently unconformably) the typical Pale Shales 

 of the Rhayader district. It now seems perfectly clear, however, that this 

 upper group is entirely absent from the neighbourhood of Rhayader. 



' Another point in striking agreement with the same results is the following:: — 

 The highest point in the Rhayader district — the summit of the mountain Moel 

 Hywel — does not lie in the older and more arenaceous rocks of the Upper or 

 Lower Llandovery Series, but in the very heart of the softer Rhayader Pale 

 Shales. Nothing is exposed on the summit of the hill ; but a pile of grit-blocks 

 in the cairn, and grit-debris lying around, shows that the mountain must be capped 

 by arenaceous rocks. This doubtless corresponds to the base of the Talerddig- 

 Grit group of Tarannon : for, immediately below the summit of this hill, the 

 typical graptolites of the Monograptus-crispus zone can be found in the local 

 quarries. 



' South of the Rhayader district, in the neighbourhood of New Bridge, only 

 the bright-coloured mudstones are exposed, and have been mapped as Tarannon 

 by the officers of the Geological Survey. The absence of a similar set of beds in 

 the Rhayader district was always puzzling to my mind ; but the researches of 

 the Authoress show conclusively that the ' Tarannon ' of Rhayader and the 

 ' Tarannon ' of New Bridge are not of exactly the same age. 



' Another point is the absence of an arenaceous base to the Tipper Llandovery 

 Series in the Tarannon district, and it would appear almost as if there were 

 a complete passage upwards from the DimorpJiograptus-Swanstoni to the 

 Monograptus- Sedgwickii Beds. This, however, differs from the succession at 

 Rhayader, where there is a distinct unconformity between the Upper and the 

 Lower Llandovery Groups, with a base to the Upper Llandovery of several 

 hundreds of feet of massive grits and conglomerates. I understand from the 

 paper that the Authoress has not discovered an unconformity ; but, as the lower 

 rocks are exposed only in patches, it is quite possible that an unconformity may 

 yet be found. 



'As to the inclusion of Rastrites maximus with the Taraunon Shales, there is 

 no reason, from the evidence in the Rhayader district at any rate, why this 

 should not be adopted, although I selected Monograjjtus exiguus myself as 

 a suitable fossil for the first Tarannon zone. The Pale Shales of Rhayader 

 commence at the Eastrites-maximus Beds, and run on without a break up to 

 the summit of the Monograptus-crispus Group : so that, if one adopted the 

 Authoress's suggestion, the Upper Llandovery would be represented by the 

 grits and conglomerates of Caban Coeh, and the Rhayader Pale Shales would 

 lie wholly in the Tarannon Series. 



' Lastly, the enormous thickness of the Tarannon Series in the Tarannon 

 district will bear out the conclusions already formed with regard to the same 

 rocks at Rhayader, where only the lower half is exposed ; and it has been 

 pointed out that the thickness of even this lower half must be very great, for it 

 covers a large area, extending some 12 miles to the north, and about the same 

 distance to the east — altogether not less, probably, than 150 or 200 square miles. 



' In conclusion, it gives me great pleasure to be one of the first to con- 

 gratulate the Authoress on a most interesting and valuable piece of work.' 



Mr. W. G. Fearnsides further congratulated the Authoress upon 

 the completion of the fine piece of zonal work described in the 



