478 PROP. T. G. BONNEY ON ROCKS IN 



27. On the Bocks between the Quartz-felsite and the Cambrian 

 Series in the Neighbourhood of Bangor. By T. G. Bonnet, 

 M.A., F.R.S., Sec. G.S., Professor of Geology in University 

 College, London, and Fellow of St. John's College, Cambridge. 

 (Eead May 23, 1883.) 



This district has already been the subject of papers published in the 

 Quarterly Journal of the Society *. The authors of these agree in 

 accepting the coarse conglomerate exposed (for instance) on the 

 upper part of the hill pierced by the eastern tunnel of the railway 

 near Bangor, as the base of the Cambrian ; but there is still some 

 diversity of opinion as to the relations and extent of the sub- 

 jacent series. The views on the latter point will be found respec- 

 tively expressed in papers by myself and Professor Hughes in vol. 

 xxxv. ; and since the publication of the latter P have four times re- 

 visited the district, leaving, T believe, but few exposures of the rock 

 unexamined (except such as may occur unknown to me in private 

 grounds). As I adhere to my original opinion as to the sequence 

 of the beds, I now ask leave to state as briefly as possible the addi- 

 tional evidence which I have succeeded in gathering. At the same 

 time I am glad to take this opportunity of frankly admitting that 

 in the part of his paper which treated of the relations of the con- 

 glomerate and the granitoid rock at Twt Hill I was wrong, and 

 Prof. Hughes was substantially right. Repeated examination of 

 the district and of specimens under the microscope has convinced 

 me that, notwithstanding the apparent passage of the one into the 

 other, the conglomerate is much later in date than the granitoid 

 series. Whether it be of the same date as the Cambrian conglo- 

 merate in the Bangor area is, I think, at present by no means certain. 

 I can hardly regret the mistake, for in the correction of it (as re- 

 lated in Geological Magazine, decade 2, vol. ix. p. 18) I have learned 

 much ; and that I made it at all was, I believe, due to the last 

 lingering impressions of the instruction which I had found in the 

 writings of those whose teaching was regarded some dozen years 

 ago as authoritative. Had I then commenced my petrological 

 studies in a spirit of absolute scepticism, I should have been saved 

 much time and some errors. 



The main point, however, at issue between myself and Professor 

 Hughes with regard to the Bangor district is the following : — I had 

 described in the district south of the fault, which runs nearly along 

 the line of the road from Bangor to Caernarvon, a series of rocks in 

 which occurred at least four well-marked beds of breccia or con- 

 glomerate, which appeared to me to be so distinct that I could only 

 conclude that they indicated horizons, and that the succession as a 

 whole was an ascending one. Three of these, unless I misunder- 

 * Vol. xxxiv. pp. 137 & 147, and vol. xxxv. pp. 309 & 682. 



