Vol. 52.] EASTERN COBBER OF ANGLESEY. 623 



III. Baron Hill or Bangor Group. 



The stratigraphical importance of this group lias been generally 

 admitted since the discovery of the classical section in the Baron 

 Hill drive by Prof. Bonney (Quart. Journ. Geol. Soc. vol. xxxi.w 

 1883, p. 470) : nor has any doubt been thrown upon his identification 

 of it with the volcanic group of Bangor. To his admirable peno- 

 logical description there is little or nothing to add. 



The stratigraphical relations of the Baron Hill outlier have, 

 nevertheless, remained in some obscurity ; and therefore certain 

 points which have come out in the course of the mapping will be of 

 some interest. 



Prof. Bonney considered it probable that the junction was 

 faulted. Prof. Blake describes (Quart. Journ. Geol. Soc. vol. xliv. 

 1888, p. 463) a visible unconformity at the south-western end of the 

 drive section. The lowest rock seen there at present, however, is the 

 ashy grit ; nor is anything else exposed in some bosses below the 

 drive at that point, which must be some feet lower down in the 

 series. Further, 1 would draw attention to the following points. 



Prof. Bonney has remarked, with justice, upon the obscurity of 

 the dips in the drive. The cuttings were then fresh, and probably 

 the bedding has since become more distinct by weathering, though 

 it is still inconspicuous. On careful examination a fine, even 

 banding in the green argillites, with occasional sandy seams, can 

 be seen, dipping steadily a few degrees W. of jS". at angles varying 

 from 30° to 90°, the average being high. Moreover, if any doubt 

 remained as to the drive section, there can be none as regards some 

 old weathered bosses along the top of the wood, where bedding with 

 the same dip and strike is perfectly dear. The thickness, measured 

 from these dips, would be not less than 1000 feet. 



It will be seen that these planes strike almost directly at the 

 N.E.-S.W. boundary (as well as at the strike of the adjoining 

 schists) ; and if the structure be considered in relation to the form 

 of the surface, it becomes evident that the outlier cannot be resting 

 undisturbed upon the schists. 



To explain this by normal faulting, however, would require very 

 large faults, of which there is no evidence whatever on the coast 

 to the S.W. ; and which, moreover, the mapping of certain zones 

 in the adjoining schists shows to be almost if not quite impossible, 

 especially when the trend of the rock-features is considered. If 

 there be any such faulting, it must, I think, be of small throw, and 

 cannot extend all along the line. With regard to the E.S.E.- 

 W.N.W. boundary, it is probable that there is a normal fault; but 

 this must be a small one, as il does not appreciably displace a 

 /one in the schists south-east of Murddyn Siglen. The northern 

 boundary, however, is almost certainly a normal fault, running 

 W.S.W. ; of which there is evidence also in th (> featuring — a deep 

 hollow in the hill-face, with a line of springs farther up. 



It appears, therefore, thai the Baron Hill outlier must be bounded 

 by and resting upon a plane of somewhat exceptional nature. 



