part 2] ITS STRUCTURE AIS'D ROCK-SUCCESSIOIf. 173 



EXPLANATION OF PLATE 11. 



Geological map of the country east and south-east of Bala Lake, on the 

 scale of 4 inches to the mile, or 1 : 15,840. 



Discussioi^. 



Prof. 0. T. Jones congratulated the Author on the completion 

 of an extremely interesting research upon a complicated district, 

 one which, as was known from his writings, drove so accomplished 

 a geologist as Jukes almost to despair. 



The paper raised many interesting questions, both of the nomen- 

 clature and structure, and particularly furnished information as to 

 the relation of the Llandeilo and Bala formations. The discovery 

 of the JSfemagraiJtus-gracilis fauna in the Derfel Limestone, asso- 

 ciated with brachiopods similar to those in the Stinchar Limestone, 

 was extremely interesting. 



In Sedgwick's final classification the base of the Bala was drawn 

 at the base of the strata overlying the ' porphyries ' of Arenig. 

 The relation of these ' porphyries ' to the Llandeilo formation of 

 South Wales was unknown. In South Wales it has been shown 

 that the Nemagraptus fauna overlies directly everything that can 

 be assigned by definition to the Llandeilo formation, and it was 

 satisfactory^ therefore, to find from the Author's new evidence that 

 the line proposed by Sedgwick for the base of the Bala in North 

 Wales coincided with the line that was adopted by the Greological 

 Survey in South Wales by direct reference to the Llandeilo forma- 

 tion. It further became clear that the terms ' Llandeilo ' and 

 'Bala ' bore very little relation to Llandeilian and Caradocian. 



He did not quite understand the Author's caution about regarding 

 a fauna of Ashgillian type in South Wales and elsewhere as neces- 

 sarily indicating Ashgillian age. The Rhiwlas Limestone of North 

 Wales, with its Phillipsinella-paraiola fauna, coincided precisely 

 Avith the beds which were taken to mark the base of the Ashgillian 

 in South Wales ; and the numerous brachiopods from rocks of Ash- 

 gillian age, which the speaker had examined from various localities 

 in North Wales, agreed closely with those of the corresponding 

 rocks in South Wales. 



The position of the Hirnant Limestone apparently still remained 

 unsatisfactory. The striped shales which succeeded it appeared to 

 correlate with the Birkhill Shales farther south, where, however, 

 they were thicker than at Hirnant, and it was also rather sur- 

 prising that they yielded no graptolites, considering their extremely 

 fossilit'erous character farther south. 



He enquired whether there was a possibility that some part of 

 the sequence was cut out by strike-faulting, in view of the existence 

 of several faults of this type ranging towards the area from the 

 south. He also asked whether there was any direct evidence of 

 great lateral movements along the line of the Bala Fault itself, as 

 seemed to be the case in the neighbourhood of Talyllyn, on what 

 was apparently the continuation of the same fault. 



