R. D. Roberts — The TidSill Conglomerate. 



115 



into a roclc which has some resemblance to the bottom, rocTc." (The 

 italics are not Prof. Bonney's.) 



In the discussion which followed, Prof. Hughes pointed out, as the 

 result of his own mapping of the district, that Prof. Bonney had 

 misapprehended the direction of the dip, having been misled by the 

 very deceptive appearance of the surface of the quarry, and had 

 mistaken the patches of conglomerate sticking to the face of the 

 quarry for alternating beds of conglomerate and granitoidite. He 

 further stated that the conglomerate really dipped away from Twt 

 Hill, passed up into brown sandstones on the S.S.E. rim of the 

 quarry, and was the basement bed of the Cambrian, resting upon 

 the Pre-Cambrian granitoid axis, as indicated in his section drawn 

 across the same ridge nearer Bangor (Q. J, G. S. Dec. 5th, 1877). 

 To make the matter perfectly clear, I give a ground-plan of the 

 quarry (Fig. 1). Prof. Bonney's section, to which the description 



Fig. 1. (Ground Plan of Twt Hill Quarry.) 



WSW 



a. Granitoid rock. 



b. Twt Hill Quartz conglomerate. 



c. Brown sandstone. 



A B. Line of Prof. Bonney's section. 

 C D. Line of Prof. Hughes' section. 



above quoted applies, was taken along the line A B. The section 

 (Fig. 2) is a copy of Prof. Bonney's, given in Q. J. G. S, vol. xxxv. 

 p. 321, and is a reduced representation of the diagram exhibited at 



Fig. 2. Prof. Bonney's section. (A to B. Ground Plan. 



A. Granitoid rock. 



B. f'ame becomino: pebbly. 



C. Coarse conglomerate. 



D. Fine conglomerate. 



-Z7. Coarse conglomerate. 



F. Finer conglomerate, in parts more like B and A. 



