484 PEOF. T. G. BONNEY ON EOCKS IN 



angular, would not seldom be more simply explained by supposing 

 that sporadic volcanic action still continued, and that indurated 

 fragments of the finer sediments formed from the denudation of the 

 somewhat older lavas and ash were ejected together with lapilli. 



I see no reason for insisting on any great interval of time between 

 the rhyolitic lavas and the base of the Bangor beds. An uncon- 

 formity there seems to be ; but that in a volcanic series is of little 

 moment ; and I am of opinion that on the whole it would be better 

 to include these quartz porphyries or quartz felsites (old rhyolitic 

 lavas) with the Bangor group. This belongs to the disturbed episode 

 anterior to the quiet subsidence which marks the non-volcanic sedi- 

 mentary series everywhere recognized as Cambrian. Prom the 

 latter we seem justified lithologically and physically in separating 

 these more or less volcanic beds, and in including them for con- 

 venience in the Pebidian group of Dr. Hicks ; but the interval in 

 time need not have been a very enormous one. Below the rhyo-r 

 lites, as it seems to me, is the great gap in the record. They appa- 

 rently broke forth, as the older basalts in Auvergne, upon a plateau 

 of crystalline rocks which belong to some of the earlier, as these 

 do to the last, chapters in the Archaean volume of the Geological 

 History. 



Note. — As I believe that no analysis has been published of any 

 specimen from the great masses of quartz -felsite, which occupy so 

 considerable an area in this part of North Wales and contribute 

 so largely to the rocks immediately overlying them, I take this oppor- 

 tunity of giving one, for which I have to thank my friend Mr. J. J. H. 

 Teall, F.G.S. It was made from a very typical specimen of the 

 common purplish variety, collected from the crag near Brithdir 

 Farm (No. I.). For comparison, I place beside it an analysis of 

 the " devitrified pitchstone " of the Wrekin district (No. II.), of a 

 felstone (of Arenig age) from Aran Mowddwy (No. III.), and of a 

 felsite of Bala age from the Lledr valley (No. IV.). The last, an 

 analysis of a specimen of the ground-mass of the peculiar spherulitic 

 rock described on p. 290 of vol. xxxviii. of the Quarterly Journal, 

 was kindly undertaken for me by Mr. F. H. Hatch of University 

 College, London, when I was engaged on my paper " On some Nodu- 

 lar Felsites in the Bala Group of North Wales ; " but the result, owing 

 to an accidental delay, did not reach me in time for publication. 



The close correspondence between I. and II. is most remarkable. 

 The Ordovician felsites (III. and IV.) agree fairly together, and 

 differ from the other pair in a larger percentage of Si0 2 , a less per- 

 centage of alumina and alkalies, and in an excess of Na a O over 

 Kfi. 



