C. A Matleij — The Arenig Bocks near Aherdaron. 121 



discovery of fossils in zone (6). The writer has not yet found 

 ' tuning-fork ' graptolites below zone (5). Both these upper zones 

 are clearly of Upper Arenig or Llanvirn age. The ironstone and 

 manganese band (4) is of stratigraphical interest, because it is found 

 again about a mile away at Tydrlyn-Meirion and at Benallt-bach, 

 Khiw. At the latter place these beds are worked commercially for 

 manganese, and the blue concretionary mudstones there have pro- 

 vided specimens of Azygograptns snecicus, Moberg, and other fossils. 

 The ironstone occurs in massive beds, and is of an altogether different 

 character from the pisolitic iron-ore, supposed to be of Arenig age, 

 found at Llanengan a few miles further east. 



W 



Tyddyn 

 Dafydd 



Maen 

 Gwenonwy 



1 / 2 3 2 2 3 



Fig. 3. — Section near Maen Gwenonwy. 



W.S.W. 



4. Drift. 



3. Dolerite Sill. 



Nant y 

 Gadwen 



B. hirimdo 



'< Ironstone 



/. Fault. 



2. Arenig rocks. 

 1. Pre-Cambrian. 



Mynydd 

 Penarfynydd E.S.E. 



Picrite and 

 J), bifidus Hornblende -dolerite 



Fig. 4. — Section from Forth Tsgo to Mynydd Penarfynydd. 



The lowest beds of this tract have not yet yielded fossils. They 

 are possibly of Lower Arenig age, and they pass upwards, through 

 Middle Arenig, into Upper Arenig beds. All the graptolites here 

 recorded are forms which also occur in the Middle Skiddaw Slates. 



CONCLTJDINa EeMARKS. 



The writer is indebted to Miss E. M, E. Wood for very kindly 

 examining and identifying the graptolites and for information con- 

 cerning their zonal value. Other fossils such as Ogygia, Trimicleus, 

 Lingula, and OrtJioceras have been obtained in several localities, but 

 their species have not yet been worked out. The graptolites, how- 

 ever, show sufficiently well that the greater part of the Arenig is 

 represented in the area described. The lowest beds of the series are 



