﻿638 THE GEOLOGY OF AEENIG EAWR AND MOEL LLYENANT. [Aug. 1905, 



passage between the Daerfawr Shales and the Upper Ashes, and 

 between the latter and the Or^ts-Limestone. This limestone the 

 Author referred to the Caradoc Series, and the fossils seemed to bear 

 out his view ; but it was distinctly a surprise to find Caradoc rocks 

 in that locality. 



Mr. A. P. Young remarked on the unsettled use of the term 

 la ceo lite, and wished to ask how far the structures studied by 

 the Author resembled those intrusive masses to which the name in 

 question was first applied. The floor of conntry-rock, the vaulted 

 roof, and the absence of any channel of communication with the 

 surface, seem to be essential characters of a laccolite, as originally 

 described. 



Prof. Hull expressed his high appreciation of the geological 

 mapping of the Author in the Arenig Mountains; and emphasized 

 the great advantage in the present day of having maps on the 6-inch 

 scale, compared with those on the 1-inch scale (which were the only 

 maps available in the old days of the Geological Survey, when the 

 district was being mapped by his deceased colleagues, Ramsay, 

 Selwyn, Jukes, and Aveline). It was under these able men that 

 he (the speaker) had his first lessons in field-geology among these 

 mountains, when he assisted in levelling the horizontal sections 

 over Aran, Cader Idris, and Snowdon. He believed that he was 

 correct in saying that the view of the Government surveyors, as 

 regarded the Arenig range, was that the great sheets of felspathic 

 trap described by the Author were in a certain sense ' con- 

 temporaneous,' and not ' intrusive,' except that they had been 

 extruded from time to time over the floor of the sea ; a view 

 which the occurrence of great beds of volcanic ash with marine 

 fossils tended to confirm On the other hand, the dykes of 

 diorite and dolerite, and other basic rocks which traversed the 

 Arenig Beds, were undoubtedly intrusive (in the ordinary sense) 

 and of later age. 



Prof. Watts compared the succession at Arenig with that worked 

 out by Prof. Lapworth in Shropshire. The main volcanic series 

 was above the zone of Didymoyraptus Murchisoni in the former 

 district, and below it in the latter. But the chief members of the 

 Arenig succession could be compared fairly closely. A strong 

 resemblance also existed between the petrological characters of the 

 andesites and the intrusive dolerites in the two areas, and there 

 were similar difficulties as to the conditions of the outpouring or 

 intrusion of the andesites. It was somewhat difficult to compare 

 the Tremadoc rocks in the two localities. 



Prof. T. McK. Hughes thought that the Society would allow that 

 circumstances had laid the task of bringing before the Society the 

 results of researches on the Arenig and associated rocks of the 

 Arenig Mountain upon one well qualified by his stratigraphical, 

 palseontological, and petrological skill to deal with it. He would 

 only add a few remarks, on some of those points as to which there 

 was less agreement. The Arenig of Sedgwick was defined to be 

 the shales tangled among the volcanic rocks of the western front of 



