The Carboniferous Succession below the Coal- Measures. 497 



the earlier mutations. The specific names must be strictly 

 kept for the special form occurring in the beds whence the type- 

 specimens were derived. We instance, more particularly, Schizo- 

 phoria resiipinata, Martin, Syringothyris cuspidata, Martin, iSpiriferina 

 octoplicatn, Sow., the types of which came from beds in Derbyshire 

 which belong to the higher coral zones. It will be a very easy 

 matter to indicate in some simple way or other the precise mutation 

 which is typical of the lower beds without introducing any 

 ambiguity. 



With regard to our researches in the Carboniferous rocks of North 

 Wales, certain very definite results obtain. We find that there is 

 no evidence that any beds belonging to the series which come on 

 below the Dibunophyllum zones in the Bristol-Mendip area are 

 represented in North Wales, except possibly some few feet of the 

 Upper Seminula beds. 



The Basal Conglomerate, containing derived fossils only, which 

 lies between the Silurian rocks and Carboniferous Limestones in 

 some localities, is unconformable to the Lower rocks, but conformable 

 to the limestones. Although red in colour it therefore represents 

 a time late in the Carboniferous period, judged from the Bristol- 

 Mendip succession, and consequently is in no way the homotaxial 

 equivalent of the Old Red Sandstone. 



The Conglomerate is due to purely phy Biographical conditions, 

 and represents the initial deposits laid down upon a sinking land 

 surface, a fact indicated by the petrological study of the pebbles 

 which enter into its composition (Strahan & Walker, Quart. Journ. 

 Geol. Soc, vol. XXXV, pp. 268-274). These conglomerates appear 

 locally on the western side of the Vale of Clwyd, and the thickest 

 section appears to be at Ffernant Dingle, where a fairly complete 

 succession can be made out. At other places, as at Minera quarries, 

 the Carboniferous Limestone rests unconformably but immediately 

 on beds of Bala age. At Llangollen Mr. Morton estimated that the 

 beds which he then called Old Red Sandstone, were 50 feet thick. 



Comparative sections of the Carboniferous Limestone show that 

 this series thins out rapidly from north to south, and while the 

 upper beds can be traced across the whole district by their zone 

 fossils, the lower beds are wanting in the south. This must have 

 been due to the fact that the southern part of the sinking land 

 surface was higher than that to the north, and consequently only 

 received the deposit later on. Hence it follows that the base of the 

 Carboniferous Limestone in North Wales does not represent the 

 same horizon in every locality, and further that the Basal Con- 

 glomerates, where present, are not necessarily all of the same age in 

 point of time, owing to the fact that deposition took place on an 

 uneven and irregular floor. 



It is questionable whether the Seminula zone of the Bristol area is 

 represented at all in North Wales. It is true that the zone fossil, 

 Seminula ficoides, Vaughan, is fairly abundant south of Dyserth, 

 between Pentre-bach and Pentre Cwm (mentioned in the Memoir 

 of the Geological Survey, "The Geology of the Coasts adjoining 



DECADE V. — YOL. III. NO. XI. 32 



