15(3 



DE. &. L. ELIEs OX THE EALA COUXTET : ""vol. Ixxviii, 



towards the top ; thev form the steep- sided valler of the Hirnant 

 below Bwlch-yr-Hwch and the Avhole of the precipitous Dallg^A•m, 

 where they are seen dipping eastwards at increasingly higher 

 angles. They occupy the greater part of Bwlch-yr-Hwch itself ; 

 but, at its south-eastern end, it is obvious from the change in 

 the scenery that the rocks alter in character, and this alteration 

 coincides with the beginning of the Poel-y-Ddinas Mudstones. 

 These beds are, in the main, highly-cleaved pale-grey mudstones, 

 but comprise some of a rubbly concretionary nature, which weather 

 in a very characteristic manner ; all dip eastwards at angles from 

 60^ to 70^, and appear to contain fossils thi-oughout, although the 

 fauna is not a rich one, the most noteworthy feature being the 

 presence of Phacops mucronaius Brongniart, the remainder being 

 chiefly small brachiopods, such as Atrypa marginalis Dalman, 



Fig. S. — S/cetcJi looJcing uortliwards, slioicing ilie Hirnant Beds 

 and their relation to tlie Silurian. (Horizontal scale : 

 6 inc]ies = l mile.) 



Pen y Dallg-wm 



Foel y Ddinas 



Bwlch yr Hwch 



l = roel-T-Ddiiias Beds. 3 = Hirnant Limestone. 5 & 6 = Graptolitic shales 



yielding grapt elites 

 indicatiTe of the 

 zones of Mono- 

 graptus sedgvsiclci 

 and 3/. crisinis. 



2= Hirnant 3Iudstones 

 below Hirnant 

 Limestone. 



4 = Hirnant Mudstones 

 above Hirnant 

 Limestone. 



Orthis (^Dalman ell a) elegantula (Dalman), SJcenidiiim sp., and 

 PlatystropJiia hiforata YRY.Jlssicosfata (Davidsen). These deposits 

 are succeeded by the Hii-nant Beds, dark-blue mudstones of much 

 softer nature, which yield far more readily to weathering than the 

 beds beneath them : they dip at a still higher angle, 70' to S0°, 

 being (as it were) plastered on the east side of Pen-y-Dallgwm. 

 They seem to occupy the head of the Hirnant valley i Cwni 

 Hirnant), and the line of the old high-level valley east of Foel-y- 

 Ddinas ; but a small tear separates the two. On Pen-y-Dallgwm, 

 due west of the shepherd's house in Cwm-yr-Aethnen, there is still 

 to be seen the old classic section in which the famous pisolitic 

 facies of the Hirnant Limestone occm-s. This is exposed in an old 

 quarry on the hillside, to which an old grass-track still leads from 

 the main Lake-Tyrnwy road. The following succession can be 

 made out there : — 



