144i DE. C4. L. ELLES OX THE BALA COrNTET : [vol. Ixxviii, 



Foel-y-Ddinas Mudstones. 



Towards their upper limit the Moel-fiyn Sandstones give place 

 to rocks of more varied character : these are mudstones with large 

 concretions, some fine-grained concretionary sandstones, and some 

 bands of a peculiar rubbh^ rock, perhaps at one time calcareous, 

 intercalated in the mudstone ; their pale-grey colour is still cha- 

 racteristic, but these beds have yielded a definite fauna of small 

 brachiopods and big trilobites {FJiaco^JS mucronatus). 



Hirnant Beds. 



These deposits are notably different in character from the 

 beds below them, showing in fact a return, as regards the mud- 

 stones, to the darker blue-grey mudstone type of the Allt-Ddu 

 Beds, but with a totally distinct fauna. The change takes place 

 fairly rapidly, and, in the tj^pical development as seen in the 

 Hirnant valley near its head, the mudstones contain a remarkable 

 pisolitic rock, the so-called Hirnant Limestone. This lime- 

 stone has only a very local development, although the pisolitic 

 mudstones associated with it are more widespread. These Hirnant 

 Beds seem to change very rapidly along their strike ; in place of 

 the mudstones and the pisolitic limestone, a series of calcareous 

 grits and purer quartzose grits occur, and this gritty character 

 appears to become more pronounced northwards. 



The highest mudstones seem to show the incoming of deeper- 

 water conditions, as fine slaty shales which form good slate-bands 

 come in, and, gradually increasing in importance, finally supersede 

 all the mudstones. Purely slaty rocks succeed, in which trial- 

 shafts have been made. These slates fall into two groups, a 

 lower blacker, harder, and somcAvhat less banded group, in which 

 Upper Birkhill graptolites have been found; and an upper softer 

 group, with some lighter bands, yielding typical Tarannon 

 graptolites. 



IT. Details oe Special Sectioxs. 

 Nant Aber Derfel. 



The nature of the Derfel Limestone and its relation to the 

 Dicranoqraptus Shales have been described and mapped by Prof. 

 W. Gr. Fearnsides, and, as he points out, the Derfel Limestone is 

 best exposed in the gorge of the Nant Aber Derfel below the 

 bridge carrying the old road from Ai-enig to Bala : here the shales 

 above the limestone seem to be entirely without any trace of 

 fossils ; but out on the moor, above the bridge, when the stream is 

 low, the shales intervening between the limestone and the Volcanic 

 Series may be seen. They have yielded the following graptolites : 

 Dicellogra])tu8 sextans (Hall), Nemagraptus sp., Climacograptns 

 schdrenhergi Lapworth, and CI. hrevis Elles & Wood. It is quite 



