340 MISS C. A. KAISIN ON THE LOWER LIMIT OF 



away, but in the part which remains by the side of the cottages 

 two greenstone dykes, three or four feet wide, are clearly exposed, 

 piercing the felsite and evidently striking towards the masses by the 

 roadside below, which I have described *. 



I examined as additional illustrations specimens, obtained at other 

 places near Llyn Padarn, of rocks which looked like slates but were 

 only examples of slaty greenstones. It is unnecessary to describe 

 their special characteristics, which would have no bearing on the 

 argument under discussion. That is admittedly founded on the 

 Bryn-Efail sections, which are said to provide a " crucial test, and 

 thus a decisive proof." These sections, however, include no slate at 

 all, and not any grit of earlier ago than the felsite. That rock does 

 not therefore mark a lava-flow of mid-Cambrian age; so Mr. Blake's 

 hypothesis breaks down, and that which has been put forward by 

 previous observers remains in possession of the field t. 



Discussioif. 



Prof. Blake said that small differences of strike in the beds 

 between two parallel conglomerates were of little importance in the 

 Bangor area ; that there was no discrepancy between his text and 

 map with regard to the tongue of felsite on the west side : and that 

 where the Arenig conglomerate was unconformable it might come 

 to lie upon any part of the lower series. He did not base his case 

 as to the Llyn-Padarn mass entirely on the section atBrynEfail. A 

 conglomerate of felsite-pebbles to the west of the mass would cer- 

 tainly be important evidence, but he had failed to find any such. As 

 to the small scattered crystals in the slaty band, he had given his 

 reasons forjudging them to have been chiastolite, though they were 

 nothing but pseudomorphs, and he had nothing to add on this point, 



* Prof. Bonnej', to whom I am indebted for aid in this paper, has given me 

 his opinion of the microscope-slides from the Bryn-Efail sections. These were 

 fourteen in number, including seven junctions, and tbej^ were sent, as he 

 requested, without labels, and mixed with slides from other localities. His report 

 on them is to the following eiFect: — ' None of them show any slate, but in ten 

 there is diabase. The junctions all suggest, and many prove conclusively, that 

 the latter rock is intrusive in the felsite. That the other rock which occurs in 

 the slides is a felsite there can be no doubt, except in one case. At first, after 

 examining this slide, I inclined, though with much hesitation, to the opinion that 

 the rock was a grit (composed, however, entirely of fragments of felsite) which 

 had been subsequently crushed; but after examining the hand-specimen, and using 

 for comparison slides in my own collection from N.W. Caernarvonshire, I felt 

 little doubt that the rock was really part of the felsite, exhibiting rather peculiar 

 flow-brecciation and subsequent mechanical crush.' 



The ten slides, which exhibit the more clearly-marked felsite, cover, as far as 

 I can understand, the grits mai-ked in Mr. Blake's diagram. The one just 

 mentioned comes from a part at the south end of the quarry, and its position is 

 shown on the plan, fig. 2, by a X. 



t See Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 137, Prof. T. McK. 

 Hughes 'On the Pre-Canibrian Eocks of Bangor;' ibid. vol. xxxiv. (1878) 

 p. 147, Dr. H. Hicks ' On some Pre-Cambrian Rocks in Caernarvonshire ; ' 

 ibid. vol. XXXV. (1879) p. 300, Prof. T. G. Bonney 'On the Quartz-felsite and 

 Associated Rocks at tlie base of the Cambrian Series in North-western Caer- 

 narvonshire.' 



