146 



WALTEK KEEPING ON THE 



vast dreary mountain-region of bare sheep-walks scantily marked 

 out into districts by poor stone walls and wire fences, with much 

 waste bog and peat land. The rocks are of uniform pale blue and 

 grey colours, varying from small papery shales (rarer) to large 

 irregular platy shale or regularly cleaved slates, also (in some areas) 

 much indurated small slate rock cross-cut into fragments by frequent 

 bedding- and joint-planes. Some zones of softer rab, like that of the 

 Aberystwyth group, and a pale mudstone rab are sometimes found, 

 especially near the junction with the grit series. 



As we reach the central mining district, some seven miles east of 

 Aberystwyth, beds of hard, pale, indurated slate-rock with frequent 

 bed-bandings occur ; and in the immediate neighbourhood of the 

 mineral veins such induration is nearly always well marked. 



Occasionally a bed of grit, 2-6 inches thick, occurs in this series ; 

 but such occurrences are very rare, so that the building- and road- 

 stones of the central " Metalliferous " country have to be carried 

 for miles, either from the Aberystwyth grit quarries in the west, or 

 from the Plynlimmon group further east. 



Thin ferruginous gritty bands, about \ inch thick, however, are 

 more frequent, some of them being highly micaceous. Many such 

 are seen between the fourth and fifth milestones and around the 

 ninth milestone on the Devil's-Eridge road. 



Hock-foldings. — Several excellent exposures of the contortions in 

 this slaty series are seen along our line of section. There is a neat 

 little synclinal at seven miles, an anticlinal towards the eighth mile- 

 stone ; and several folds may be detected about nine miles east of 

 Aberystwyth; but overriding these foldings the prevailing dip is 

 seen to be clearly and determinately to the east. 



Fossils are rare in this series. The curious branching structure, 

 Nematolites Edwardsii, Keep., occurs at Ty Llwydd ; and I have 

 found the Fan Algal (Buthotrephis major, Keep.), the Nematolites, 

 and worm-trails {Nereites) in several other places. But in other 

 areas, especially at Cwm Symlog and near Machynlleth, out of our 

 present line of section, a rich Graptolite -fauna has been discovered, 

 and will be described later on. 



The DeviVs Bridge, great Inversion of the Rocks (fig. 1). — As we de- 

 scend the hill to the Devil's Bridge we pass over pale, hard, shaly 

 slates* with thin gritty bands about 1 inch thick, marked with worm- 

 tracks {Nereites Sedgwickii) , impressions of the Pan Angal {Bathotrephis 

 major, Keep.), and Nematolites Edwardsii, Keep. Coarse roofing- 

 slates have been worked about half a mile west of the hotel. These 

 rocks belong to the Metalliferous-slate series ; but coming to the 

 waterfall we meet with a set of alternating thin grits and large 

 pale shaly slates, together with some large flags of laminated grit, 

 with fossils exactly resembling those already noticed from Cefn 

 Hendre. 



Many of the grit bands are thin and little jointed, so that large 



* Although the splitting of these rocks is so irregular and shale-like, yet the 

 planes of division are of subsequent cleavage origin ; therefore I call them shaly 

 slates rather than slaty shales. 



