458 ME. A. WADE ON THE LLANDOVERY AND [Aug. I9II. 



Discussion. 



Mr. W. G. Fearnsides congratulated the Author upon his lucid 

 exposition of the rock-succession and structure of a most fascin- 

 ating district, and upon the great advance of knowledge which 

 was represented by his results. He thought the evidence of the 

 continued instability of the Welshpool anticline during the Silu- 

 rian Period most important. It showed that the Welshpool district 

 was already unstable, even before depression had allowed Silurian 

 sediments to overlap on to the Longmynd and Carneddau anticlinal 

 districts, and that in later (Wenlock and Ludlow) times it had 

 followed rather accurately the up-and-down movements of those 

 neighbouring districts. 



In regard to the Author's correlation of his highest Ordovician 

 subdivision with the Ashgillian of Dr. Marr, the speaker was less 

 satisfied. The terms in which the Author had described the litho- 

 logical succession from the Trilobite-Dingle Group upwards might 

 well be used to describe the successive members of the Bala Group, 

 either in the region south of Bala Lake or in Western Carnarvon- 

 shire ; but, in both those districts, the place of the black shales of 

 the Gwern-y-brain Group was taken by equally black shales 

 containing Diplograptus truncatus, a graptolite which marked a 

 horizon well below that at which the great Ashgillian Series was 

 supposed to begin. In the opinion of the speaker the Ashgillian 

 Series about Welshpool must be completely overstepped by the 

 sandy and conglomeratic beds of the Llandovery. 



Mr. H. H. Thomas congratulated the Author, both on his choice 

 of ground, and on the results that he had achieved. He remarked 

 that the Trilobite-Dingle Beds, from the abundance of Ampleoco- 

 grajptus perexcavatus, represented a low horizon in the Dicrano- 

 graptus Shales, and should be succeeded by beds containing 

 abundant Mesograptus and species of the larger Diplograptids — 

 such as Orthograptus calcaratus var. vulgatus. He asked whether 

 the Author considered his Pwll-y-glo Beds to be the equivalents of 

 the higher portion of the Dicranograptus Shales, as developed in 

 other districts. With regard to the black shales above the Gaer- 

 fawr Beds, the speaker agreed with Mr. Fearnsides that there were 

 difficulties in the way of accepting them as belonging to the 

 Ashgillian. He also asked whether it were not possible for these 

 shales to belong to the zone of Pleurograptus linearis, and to repre- 

 sent similar shales which in South Wales and elsewhere were 

 characterized by Diplograptus truncatus, and occurred beneath the 

 Ashgillian. 



He was much impressed b} r the striking manner in which the 

 undoubted Llandovery beds transgressed the older series, and also 

 by the way in which high zones of the Wenlock came to rest upon 

 rocks of Valentian age. Mr. Cantrill and the speaker had in South 

 Wales also noted that near Llandeilo a high zone of the Wenlock 

 characterized by Monograptus flemingi was only a little way above 



