158 



WALTER KEEPING ON THE 



limmon mountain ; but large boulders of coarse conglomerate upon 

 Gogerddan Hill^ Aberystwyth, which surely came originally from 

 this mountain, contain casts of various fossils, of which I can men- 

 tion crinoid ossicles, Nebidipora?, Petraia, a large cup coral, and 

 Meristella . 



The thickness of this group must also be very great ; for Plyn- 

 limmon rises some 1500 feet above Garn Fach, the level where the 

 Plynlimmon grits first appear. A thousand feet is probably an 

 under-estimate. 



Part II. Brief Notes upon oilier Sections. 



(1) From Aberystwyth through Pont Erwyd to near Builth (fig. 4). — 

 Prom Aberstwyth to Pont Erwyd the section is similar to that from 

 Aberystwyth to the Devil's Bridge ; but the western dips are more 

 important towards Pont Erwyd, so that the inversions required are 

 here smaller or less numerous. Beyond Pont Erwyd is a great slate 

 and shale series running for many miles, nearly to Ehyader, and 

 forming by the apparent dips an important synclinal under the highest 

 ground south of Plynlimmon. Eossils occur at Pont Erwyd and 

 Dyffryn Castell, also near Ehyader. The hard pale-slate series comes 

 in at Goginan and east of Pont Erwyd, and there is a similar rock 

 west of Ehyader ; but the Plynlimmon grits do not actually appear 

 as such, unless the grits of Llynodd leuan belong here ; they are, 

 however, well developed in the mass of Plynlimmon farther north. 

 Some of the paler slate along our line of section may represent this 

 series. Grit beds similar to those of the Aberystwyth group are 

 found in two places near the ' Glansevern Arms,' and again further 

 on east of Langwrig. They are to be regarded as special local de- 

 velopments in the Metalliferous Slates. Other grits and conglomerates 

 seen at Gwastaden are probably the representatives of the Plyn- 

 limmon grits. 



(2) Section from Llandeilo to Aberaeron. — Three principal grit- 

 areas are met with in the line of this section — namely, at Talieris 

 (west of Landeilo), east of Lampeter, and at Aberaeron. The Talieris 

 conglomerate beds are surmounted by dark shales and shaly slates 

 of the Metalliferous types ; and beds of the Ehyader pale-slate type 

 next underlie the grits of the country east of Lampeter. By an 

 exceptional appearance the Metalliferous slates of the west seem to 

 underlie the Aberaeron grits. No pale slates are seen beneath the 

 Talieris conglomerate beds. The principal axial fold in the section 

 seems to be a synclinal in the Teifi valley by Lampeter — the 

 mountains to the east and west of this being (that is, if the rock- 

 dips have any truth in them) great anticlinals in the Metalliferous - 

 slate and Ehyader Pale -slate series. The beds of grit and con- 

 glomerate at Talieris are not of the nature of basement beds of a 

 stratigraphical group, but indicate nothing more than such slight 

 physical variations as a shallowing of the sea-bed or a change in the 

 direction of the currents, resulting in the formation of sand and 

 pebble banks ; for the pebbles are not fragments of the underlying 

 rocks, and there is no trace of any physical break or even change of 

 lithological character above and below them, 



