Frof. T. G. Bonneij — The Tiot-Kill Conglomerate. 21 



and identify, so far as one can venture to do this, with that of the 

 luass which begins near Port Dinorwic. 



This is to my mind a conclusive argument, and Dr. Eoberts may 

 exclaim, Suo sibi gladio Jiunc jugulo ; for I should certainly have 

 declined to die by his weapon. The lesson, however, is a valuable 

 one, for it shows how cautious it is needful to be in dealing with 

 these ancient rocks, and above all it proves the impossibility of 

 settling questions relating to them without a thorough microscopic 

 study. 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 for this reason to listen to certain doubts suggested by 

 my microscope, which led me wrong, as it has done other observers ; 

 it was my microscope, and that alone, which set me right ; for on the 

 afternoon when I left the section near Cefn Cynrig I fully thought 

 I should obtain from the rock a verdict in my favour. 



From every point of view the rarity of these felsite fragments is very 

 singular at Tan-y-maes. Where the two rocks are almost in contact 

 I have not yet discovered them ; ^ yet in all the other conglomerates 

 over a wide area in the Bangor-Carnarvon district they abound. 

 I may add that I prefer at present to regard the exact age of these 

 Twt Hill conglomerates as uncertain. There is no doubt much to 

 be said in favour of regarding them as a local development of the 

 Cambrian conglomerate ; but, as I am convinced this is not the only 

 one in the district, there is something to be said in favour of assign- 

 ing them to a rather earlier date. The matrix certainly is more altered 

 than is usual with the true Cambrian beds. 



But, while abandoning the view expressed in the paper, written 

 conjointly with Mr. Houghton, as to the Twt Hill conglomerate, I 

 wish to state distinctly that I do not recede from the opinion 

 expressed in the preceding communication^ and in this Magazine,^ 

 as to the age and relation of the various conglomerates on the 

 north-eastern edge of the felsite in the vicinity of Bangor. The 

 whole of that district was again worked over, last year, with the 

 result of obtaining much additional evidence and unhoped-for con- 

 firmation of my views as to the presence of a considerable series of 

 beds (conglomeratic, slaty, etc.) between the felsite and the true 

 Cambrian conglomerate. As regards this district, and as regards 

 the opinion expressed concerning certain rash and unscientific 

 generalizations about the Anglesea schists, I adhere to the views 

 which I have already published, and may hereafter ask permission 

 to give my reasons.^ It will, however, be some time before the 



1 Further examination of specimens makes me doubtful if this is more than an 

 accident owing to the state of the rocks ; for I find some rotten fragments which may 

 be felsite. Hence the section at Tan-y-maes (figm-ed on p. 301, of Vol. VII. of 

 this Magazine) does not bear out the conclusion which from it seems more probable. 

 I may also mention that I find from further examination that all the rock exposed in 

 the bank under the cottage gardens is a spherulitic felsite. 



2 Quart. Journ. Geol. Soc. vol. xxxv. p. 309. 



3 Dec. II. Vol. VII. p. 298. 



* It will be prudent not to accept the " Crug beds " as an upper subdivision of the 

 " Carnarvon series " till they have been further examined. Perhaps it may be found 

 that the felsite has been at some of its old tricks in this district also. 



