LIFE-ZONES IN THE BRITISH CARBONtPEHOttS ROCKS. 803 



In the liiuestoncs which iiuuiediately succeed the Eatieiiieut beds the 

 thief fossils of iniportcaiice are Davieaieffa Uaiu/oUeiisi^, and Sevnuula aff. 

 *icoidra. The former characterises the lowest limestones, and for North 

 Wales is a 'very satisfactory zonal index. Sinninnla att". ficoidea is im- 

 portant Vjecause it enables us to con-elate the beds with the Bristol area. 

 Only some very few feet of rocks contain this fossil, but we got it at 

 Tilysvaen and at the base of i^foel Hiraddug, in a small quarry between 

 Pcntre Cwm and Pentre Bach. Here we obtained plant remains, amongst 

 which INlr. Kidston recognised ArcJutosiyillaria Vatmxeiin, Sopp. In 

 Great Britain this plant hitherto was only knoAvn to occur in the 

 Carboniferous Limestone near 8hap Toll Bar. 



The presence of only a few feet of beds containing >'i'e)tiinula afT, 

 ^ficoidea at once fixes the horizon of the lowest limestones of North Wales 

 as the topof the Upper Semimda zone and base of the lower Dibiuiophijllum, 

 and unmistakably demonstrates the absence of all the zones of the Bristol 

 .Sequence below. Consequently in North Wales there are no representa- 

 tives of the Tournaisian, or Lower Carboniferous. W^e also learn that 

 the North Wales area was not submerged till late on in Carboniferous 

 Limestone times, and that the Basement beds are probably of Upper 

 Seminula age. 



Above the Seminula beds are a series of limestones with Cyalho- 

 ])1ujlhun Murchisoni, Bibunuphi/Uum, ({>, and a number of species of 

 Brachiopoda highly suggestive of the Lower Dibunophyllum zone. 



This zone we were able to trace south to Llangollen, and thence on to 

 Perth y Waen, near Llanymynech. 



Higher up in the Limestone Series is a very rich Upper Dibunophyllum 

 zone in which all the corals occur, which Dr. Vaughan regards as typical 

 of that zone at Bristol. The characteristic forms are Dibunophyllutn, tp, 

 Lo7isdaleiaJlo7'iformi.<t, L. rugosa, Lithostrofion jimcenni, L. irregidare^ 

 Cyathophyllum reijium, PJiilUpsastrea radiata, Campophylhim MiircJtisoni\ 

 In this zone Productus giganteus and its variant and allied forms occur 

 in profusion and in some localities, AVaenbrodlas, Llangollen, a rich and 

 varied brachiopod fauna is also present. 



We were able to mark off at the top of the Upper Dibunophyllum 

 zone certain beds which contained the corals Amplexi-rMphrentis and 

 Cijafhaxonia, and which we wished to denote as a sub -zone of the Upper 

 Dibunophyllum. We do this because we have noted the occurrence of 

 this zone immediately underlying the base of the Pendleside Series in 

 North Staffordshire and throughout the Craven and Bolland districts 

 in Yorkshire. At Whitewell the shales Avhich immediately succeed the 

 Carboniferous Limestone are crammed with these corals, and at the 

 Cracoe Knolls, beds with these coi-als are seen to intervene between the 

 Posidonomyabeds of the Pendleside Series and the Upper Dibunophyllum 

 zone of the limestones. This sequence is seen in a little stream south- 

 east of the hill called Skelterton, 



In North Wales in certain localities the colour of the limestones and 

 tiieir lithological character vary considerably, and the abundance of 

 fossils is very variable. 



Locally certain dark limestones in the Upper Dibunophyllum zone 

 are known as Aberdo limestones, and were once extensively worked fot* 

 hydraulic cement. 



Higher up occurs another series of black limestones, but lithologically 

 the two series are vastly different in fracture and composition ; but more 



