R. D. Roberts — The Twt-Hlll Conglomerate. 117 



upon the following grounds, as summarized by him in Geol. Mag. 

 Jan. 1882 :— 



1. His inability to detect any line of demarcation between the 

 granitoid beds and the conglomerate. 



2. The absence of felsite pebbles such as occur in the Cambrian 

 conglomerate of the Bangor district. 



. 3. Microscopical evidence — the greater metamorphism of the 

 matrix of the Twt Hill conglomerate than is usual in the Cambrian 

 conglomerate. 



In my two short papers to this Magazine on this subject these 

 points have been dealt with. 



1. I examined with great care on several occasions the junction 

 of the conglomerate and granitoidite in the Twt Hill quarry, and 

 came eventually to the same conclusion as Professor Hughes and Mr. 

 Tawney — the conclusion in which Professor Bonney now concurs — 

 viz. that the conglomerate and granitoidite do not belong to the 

 same series. 



2. To meet Professor Bonney's argument founded upon the litho- 

 logical dissimilarity of the Twt Hill and Bangor conglomerates, I 

 pointed out that if in a neighbouring area, a precisely similar quartz 

 conglomerate, loitliout felsite pebbles, constituted the base of the 

 Cambrian, it was at any rate conceivable that the Twt Hill con- 

 glomerate might be Cambrian, despite the absence from it of felsite 

 pebbles. The inference that the Twt Hill conglomerate was not 

 Cambrian because it contained no felsite pebbles, implied the major 

 premiss : " Cambrian conglomerates always contain felsite pebbles." 

 Professor Bonney's own words on this point are, "I have never yet 

 seen a felsite pebble in this conglomerate, or detected a fragment 

 under the microscope, yet that rock abounds in every known exposure 

 of the Cambrian conglomerate.'''' ^ (The italics are mine.) I denied 

 Professor Bonney's major premiss on the ground that the Cambrian 

 conglomerate of a part of Central Anglesea^ consists like the Twt 

 Hill conglomerate of quartz pebbles imbedded in a felspathic 

 matrix. Professor Bonney is mistaken in supposing that I regarded 

 a "felspathic matrix" as identical with "fragments of felsite" ; my 

 argument was of an entirely different nature : that I am sure is 

 clear without further words. Professor Bonney's argument therefore 

 against the Cambrian age of the Twt Hill conglomerate based 

 upon the lithological dissimilarity of the Bangor and Twt Hill 

 conglomerates has fallen to the ground, because his major premiss 

 cannot be maintained. 



3. Microscopical evidence was not neglected. I got sections made 

 of several of the rocks, and referred to some of these in my papers, 

 but the evidence thus obtained was not sufficient to solve the 

 problem. 



My reason for believing the Twt Hill conglomerate to be Cam- 



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



2 There is no question that these Anglesea conglomerates are Camhrian, for Prof. 

 Hughes has mapped them over the whole area, and found them in some sections to 

 be fossUiferous, and to pass up into fossiliferous sandstones. 



