Yol. 55.] ANP TUFFS IS-EAU CONWAY. 175 



segregated into cumulites. In other fragments the devitrification 

 is partly microlitic and partly globulitic. There are also a few very 

 obscure indications of a small spherulitic structure, but in no 

 instance is there any trace of perlicity. 



Using the more recent nomenclature, the lavas described in 

 this paper would be termed eorhyolites by some petrologists, 

 aporhyolites by others. I have, however, simply employed the 

 name rhyolite for such lithoidal rocks as may originally have had 

 either a lithoidal or vitreous character. Since they are now lithoidal, 

 they differ in no respect from recent rhyolites or lithoidites, but in 

 cases where the rock was once an apparent obsidian I have called it 

 a devitrified obsidian. All of these rocks are embraced under 

 the general term felsitic lavas. 



Between the two nodular beds, no fed by Prof. Bonney, it is at 

 present impossible to say how many similar beds, if any, occur, 

 but a more minute investigation of this intermediate area might 

 yield interesting results. 



The foregoing paper must be regarded merely as a small appendix 

 to Prof. Bonney' s work in the vicinity of Conway, but I trust that 

 it may serve to confirm some of bis statements. The observations 

 which he has made upon the felsites of Diganwy and their relation 

 to those of Conway Mountain open up questions which can only be 

 solved by mapping those portions of the district on a larger scale 

 than that of the 1-inch map. 



Discussion. 



Prof. Bonney expressed his gratification at the Author's return 

 to work, and was glad to find that he did not differ materially 

 from the results which the speaker had obtained several years ago. 

 He quite agreed that it Avas sometimes very difficult to distinguish 

 between fragmental acid volcanics and acid lavas with flow- 

 brecciation. He distinctly remembered the occurrence of nodular 

 felsites at Diganwy, but had not attempted to work out the strati- 

 graphy, because he was investigating a special question, namely, 

 the origin of the nodular structure. The Author had evidently 

 studied a set of rocks closely resembling those examined by the 

 speaker, and the latter thought it possible that he might have 

 touched one end of the Author's section. 



Lieut. -Gen. IIcMahon congratulated the Author on his lucid and 

 interesting paper. He agreed with him that it was sometimes 

 extremely difficult to distinguish between a lava that had caught up 

 numerous fragments of an ash, and an ash-bed the fine matrix 

 of which had been much altered by aqueous infiltration or contact- 

 metamorphism. In such cases a large series of specimens was 

 necessary, to enable the petrologist to ascertain with any confidence 

 the true character of the rock. 



The Peesident, Mr. Haekek, and Mr. Steahan also spoke. 



The AuTHOE, in reply, said he was glad to find that Prof. Bonney 

 was able to confirm many of his statements. It was to his 



