118 R. D. Roberts — The TwtSiU Conglomerate. 



brian is the stratigraphical evidence given in Prof. Hughes' papers, 

 and verified in the field. Prof. Bonney now accepts that con- 

 clusion, because he has detected vs^ith the micx'oscope felsite frag- 

 ments in the grits at Cefn Cynryg. This is the grit of Careg Goch 

 described by Prof. Hughes, and shown in his Map and sections. It 

 Tnust be carefully noted that these felsite fragments have not been 

 detected in the Twt Hill conglomerate, but in the grit of Cefn 

 Cynryg or Careg Goch. 



This grit of Cefn Cynryg is four miles distant from the Twt Hill 

 conglomerate, and the assumption of the identity of the beds 

 obviously depends on stratigraphical evidence. A year and a half 

 ago Prof. Bonney wrote of the Twt Hill conglomerate as follows : — 

 " Even ' if it were proved that this conglomerate wei'e made up from 

 the materials of the rock on which it rests, I could not believe it to he 

 of Cambrian age, because the microscope shows that its matrix and the 

 finer bands intercalated with it are m,uch more highly altered than is the 

 case with the Cambrian roclts." (The italics are mine.) That state- 

 ment is explicit, and indicates a perfectly definite and final opinion. 

 He now writes^: "While repudiating Dr. Eoberts' arguments, I 

 admit the substantial correctness of his conclusions as regards the 

 Twt Hill conglomerate," and further on adds : " It was trusting too 

 much to my eyes in the field (though they were perhaps not less 

 experienced than those of Dr. Eoberts), and refusing to listen to 

 certain doubts suggested by the microscope, which led me wrong, as 

 it has done other observers ; it was my microscope, and that alone, 

 which set me right." Is not Prof. Bonney mistaken in this ? It 

 was his microsccpe which led him wrong, when it led him to the 

 conclusion above quoted that the Twt Hill conglomerate could not 

 possibly be of Cambrian age. The stratigraphical evidence, by 

 leading him to infer that the strike of the rocks was different from 

 that which he was first inclined to believe, has set him right by 

 showing the possibility of regarding the Careg Goch grits near the 

 C of Cefn Cynryg, and the Twt Hill conglomerate, as "a local 

 develo]3ment of the Cambrian conglomerate." It does not appear 

 that the microscope has revealed anything new as to the structure 

 of the Twt Hill conglomerate since the above italicized paragraph 

 was written ; it has merely shown that the Cefn Cynryg grit, four 

 miles distant, contains felsite fragments, a fact which is scarcely of 

 first-rate importance, seeing that the felsitic character of the Cam- 

 brian conglomerate at Llanddeiniolen, which has been identified 

 Avith that at Careg Goch, has been known all along. I make 

 no further comment. The points brought out in Prof. Hughes' 

 papers on North Wales are now, therefore, thoroughly established, 

 viz. that the succession of the Cambrian is as follows : (1) A base- 

 ment conglomerate series, consisting sometimes of a quartz con- 

 glomerate, sometimes of a felsite conglomerate, and sometimes of 

 quartz or felspathic grits, succeeded by (2) brown sandstones, often 

 fossiliferous, and (3) black slates. 



1 Geol. Mag. Dec. II. Vol. VII. p. 300. 



2 Geol. Mag. Dec. II. Vol. IX. p. 20. 



