168 



WALTER KEEPING ON THE 



grits is clearly marked over a large area, extending from near Borth, 

 through Aberystwyth, and southwards to Llangrannog *, a gradual 

 transition between the two groups being everywhere seen. And that 

 the rocks here occupy their original relative positions is proved by 

 the position of the raised rock-markings, which are uniformly found 

 upon the under surfaces, as is normal in the lower palaeozoic rocks, 

 whereas when the beds are inverted these structures are found 

 upon the upper surface. 



The only plan of exact harmonization would be, it seems to me, 

 to suppose that the Aberystwyth grits had died out to the east 

 before we reach the fossiliferous beds of Morben and Cwm Symlog, 

 and older beds had been somehow brought up in the confusion of 

 folds and synclinals. This may be considered possible ; but it 

 again increases our difficulties in the eastern area (Devil's Bridge 

 &c), where the usual order of the strata persists. I therefore 

 adhere to my original classification as given in the earlier pages of 

 this work ; for, indeed, the palseontological difficulty is very insig- 

 nificant. 



Comparing the faunas of the Aberystwyth grits and Metalli- 

 ferous slates, we find that these do not afford any important 

 data for their separation, but rather demonstrate their intimate 

 connexion. Twelve of our species of Graptolites are known only 

 from the Metalliferous slates — namely Bastrites maximus, JR. pere- 

 grinus, Monograptus spinigerus, cyphus, gregarius, intermedins, 

 runcinatus, and involutus, and all the Diplograpti : but this signi- 

 fies probably little more than the less favourable conditions for 

 Graptolitic life in the Aberystwyth grits ; for the two rock-groups- 

 are bound together by the common possession of Monograptus dis- 

 tans, Glingani, Sedgeivickii, crenularis, spiralis, lobiferus, Hisingeri, 

 and tenuis, and Climacograpsus scalaris. The OrtJioceras, Nereites, 

 Fan- Algae (Buthotrepliis), and Nematolites (?) tubidaris are also 

 common to the two groups, and serve further to demonstrate the 

 palaeontological unity of the Aberystwyth and Metalliferous series. 



But there is another group (the Plynlimmon group) whose exact 

 age has not yet been fully considered. These beds contain no fossil 

 evidence in themselves ; but still, from their relations to the under- 

 lying metalliferous slates and to the more eastern sections, there is 

 little room for hesitation as to their stratigraphical position. 



The underlying rocks are shown to belong to the highest part of the 

 Lower Llandovery group, or Upper Birkhill series, some of the species 

 being actually of Gala types. Paralleling, then, our rocks with 

 those of the south of Scotland, this overlying group of grits must 

 either be close upon or actually in the Gala and Tarannon group. 

 "We have already seen how the stratigraphical and lithological con- 

 siderations support the view that they really are an arenaceous 

 development of the lower part of the Tarannon shale, thus being 

 the true representatives of the Gala group of Scotland. The asso- 

 ciated pale slates of Plynlimmon and around Ehyader (Ehyader pale 



* There is a difficulty, as before mentioned, at Aberaeron. which I have not 

 been able thoroughly to investigate. 



