GEOLOGY OF CENTKAL "WALES. 



167 



species are nearly equally distributed in the two zones (tenuis- and 

 argenteus-zonea) of that area. Also, amongst the other fossils, Mr. 

 Marr recognizes the fragment (tail) of a Phacops, found by him at 

 the Devil's Bridge, as a new species also occurring in the " Mud- 

 stones " of the Lake District. 



Lithologically we find some of the Graptolite-bearing beds not 

 unlike part of the Coniston Mudstone series; and some paler 

 shales aud slates, associated with the Graptolite beds at Moreen, 

 Machynlleth, and occurring again at Cwm Syrnlog, have their repre- 

 sentatives also in the same series. Still, regarding them as a whole, 

 our Cardiganshire rocks decidedly do not resemble their representa- 

 tives in the English lake country. 



"We have seen that our various fossil lists indicate that all the 

 fossiliferous beds belong to one general horizon. Certain minor 

 differences, however, are to be observed, of which the most important 

 are the presence of Mo nograptus turriculatus at Cefn Hendre and the 

 Devil's Bridge, and the species of Cladograpti of the latter place, 

 which are common to the Gala group of Scotland. 



Prom the Graptolitic evidence Mr. Lapworth would arrange the 

 beds in the following order : — 



D. The M. turriculatus beds of the Devil's Bridge. 

 C. Next below (or perhaps identical with (D) ) the Cefn Hendre 

 grits. 



B. Then comes the rich Graptolitic zone of Morben and Cwm 

 Symlog. 



A. Lowest of all lie jointly the Diplograpsus beds of Taren y 

 Gesail, Corris, and Steddfa Gurig. 



It will, however, be seen that this order does not correspond with 

 our well-established succession of the greater groups as given in 

 the first pages of this paper ; for it would make our Metalliferous 

 slates newer than the Aberystwyth grits. Now, with all my general 

 confidence in fossil evidence, I consider that the palaeontological data 

 are here insufficient to maintain this order against the very strong 

 stratigraphical evidence to the contrary. The principal fact is the 

 occurrence of a single species (Monograjptus turriculatus) in certain 

 localities ; and this does occur, only less commonly, in the Birkhill 

 shales as well as the Gala group. Moreover the similar (more 

 arenaceous) physical conditions common to our Devil's-Bridge bed 

 and the Gala group may have to do with their having some fossils 

 in common. 



Mr. Lapworth' s researches have established the fact that, in the 

 Silurian and Cambrian periods, zones of Graptolites characterize defi- 

 nite stages, just as do the Ammonites in the Jurassic rocks. But these 

 latter have their exceptions, and the minor zones are only trust- 

 worthy within local limits ; much more so should we expect this 

 to be the case with the more lowly organized group of Graptolitidse. 

 The occurrence of Monogr. colonus with Birkhill species in IN". Wales 

 is, indeed, an example of such. On the other hand the stratigra- 

 phical succession of the Metalliferous slates over the Aberystwyth 



n2 



