20 Pro/. T. G. Bonney — The Twt-JSill Conglomerate. 



look for these in the outcrops near to Port Dinorwic, which have far 

 more bearing on the question ? So far as I can discover from his paper, 

 he has left them unvisited. Nevertheless, in a matter vrhere the 

 evidence before one can only be imperfect, it is possible to get some- 

 times to a right conclusion by a wrong method, and sometimes to a 

 wrong conclusion by a right method ; and in this case I must frankly 

 confess that, while repudiating Dr. Eoberts' arguments, I admit the 

 substantial correctness of his conclusion as regards the Twt Hill 

 conglomerate. I have, however, altered my views for reasons which 

 are more satisfactory to my own mind than any which he has 

 advanced. They are: (1) because from prolonged study of the 

 microscopic structure of metamorphic rocks, I have become more 

 doubtful of the possibility of two such beds as the granitoid rock 

 and the conglomerate belonging to one series ; and (2) because I 

 have at last found felsite fragments in the conglomeratic series in the 

 Carnarvon district. 



In September, 1880, Mr. Houghton conducted me to a pit (found 

 by him subsequent to the date of our paper) which afforded the 

 section diagrammatically represented below, about half a mile 

 S.S.W. of Tan-y-maes. It is at a spot on the map about two- 

 tenths of an inch south of the first C in Cefn Cynryg, and there 

 can be little doubt of its belonging to the same series as the grits 

 near Careg Goch and Tan-y-maes. 



Diagrammatic Section of Pit near Cefn Cynryg. 



I copy the annexed description from my note-book : — " The lowest 

 beds visible (a) are distinctly bedded quartz-grits or quartzites con- 

 taining sometimes quartz pebbles up to the size of very small peas ; 

 then the grit becomes a little more felspathic and passes through an 

 intermediate stage (6) into a band about half a yard thick, which is 

 a quartz-felspar crystalline-looking rock (c), not unlike that of Twt 

 Hill, but which on the weathered surface, and sometimes even on 

 the unweathered, shows now and then distinct little quartz pebbles 

 in size from a hemp-seed to a small pea. Then comes a greenstone 

 dyke [d) — very rotten — about a foot or so thick, on the other side of 

 which the rock is again a distinct grit (e)." The dip is roughly 

 35° S.S.E. Specimens were selected from all these beds and cut for 

 microscopic examination. I fondly hoped that bed (h) would give 

 me the evidence for which I had long been seeking and establish the 

 correctness of my conclusion as to the Twt Hill group, besides 

 supplying me with helpful matter as to other work. I regret to say 

 that — exactly as sometimes happens with the felsite itself — the grani- 

 toid aspect proved illusory, and in the slide I found one or two small 

 fragments that I must recognize as felsite {i.e. devitrified rhyolite) 



