98 PROCEEDINGS OF THE GEOLOGICAL SOCIETi'. 



slowly expiring vents of this region. ISTo more striking evidence 

 could be desired of a gradual extinction of volcanic action. Through 

 many hundreds of feet of strata which now supervene, representing 

 the closing ages of the Cambrian and the earlier ages of the Silurian 

 period, no trace of volcanic material has been found in this district 

 until we reach the Bala lavas and agglomerates of Snowdon and 

 the Pass of Llanberis. 



In the neighbourhood of Bangor another area of similar rocks 

 wraps round the northern end of the western porphyry ridge. The 

 Geological-Survey map, in conformity with the ideas that governed 

 its representation of the older rocks of Anglesey and Caernarvon, 

 colours these as altered Cambrian. That this error should have 

 been made, or, when made, should not have been speedily corrected, 

 is all the more surprising when we consider the thorough mastery 

 which the surveyors had acquired of the aspects and the interpre- 

 tation of ancient volcanic rocks in Wales, and when, moreover, we 

 remember that as far back as 1843, long before the Survey of Caer- 

 narvonshire was published, Sedgwick had pointed out the true 

 volcanic nature of the rocks. That great pioneer recognized the 

 presence of '^ trappean conglomerates " and " trappean shales 

 (Schaalstein) " among these deposits at Bangor; but he could not 

 separate them from the Cambrian series of the rest of Wales *. 

 And in his section he represents them as undulating towards the 

 east and passing under the great mass of the Caernarvon slates and 

 porphyries. 



This interpretation, which I believe to be essentially accurate, 

 was modified by Professor Hughes, who, fixing on a conglomerate 

 as the base of the Cambrian system, regarded all the rocks below it, 

 or what he termed his " Bangor group," as pre-Cambrian t. He has 

 been followed in this view by subsequent writers % ; but Mr. Blake 

 has more recently argued that here, as in the Llanberis district, 

 there is no evidence to separate the volcanic detrital deposits above 

 the porphyry from the Cambrian system §. 



A little southward from Bangor the quartz-porphyry is overlain 

 by a most interesting group of fragmental rocks, the " Bangor 

 group " of Professor Hughes. These are largely of volcanic origin ; 

 they must be some hundreds of feet thick, and pass under the 



* See Proc. Geol. Soc. vol. iv. p. 212; Quart. Journ. Geol. Soc. vol. iii. 

 (1847) p. 136. 



t Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 137. 



J Bonney, op. cit. vol. xxxv. (1879) p. 316 ; Hicks, ibid. p. 296. 



§ Op. cit. vol. xliv. (1888) p. 278. 



