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 interpretation of ancient volcanic rocks in Wales, and when, moreover, we remember that as far back as 1843, long before the Survey of Caernarvonshire 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.[115] And in his section he represents them as undulating towards the east and passing under the great mass of the Caernarvonshire slates and porphyries. [Footnote 115: _Proc. Geol. Soc._ vol. iv. p. 212; _Quart. Journ. Geol. Soc._ vol. iii. (1847), p. 136.] 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.[116] He has been followed in this view by subsequent writers;[117] but Mr. Blake has 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.[118] [Footnote 116: _Quart. Journ. Geol. Soc._ vol. xxxiv. (1878), p. 137.] [Footnote 117: Prof. Bonney, _op. cit._ vol. xxxv. (1879), p. 316; Dr. Hicks, _ibid._ p. 296.] [Footnote 118: _Op. cit._ vol. xliv. (1888), p. 278.]