170 Rev. J. F. Blalie — The Llanheris Unconformity. 



History of Previous Opinion. 



The older view expressed by Sir A. Eamsay in liis " Geology of 

 North Wales"' (A) is too well known to require much explanation. 

 He regarded the felsite here exposed as a single mass which had 

 intruded into, and in places completely altered, the adjacent con- 

 glomerate. He based his view principally on the fact that the 

 matrix of the conglomerate closely resembles the felsite, even to 

 the porphj'ritic crystals, the passage between the two being quite 

 gradual. We must not suppose that such a geologist as Sir A, 

 Eamsay was misled in this matter, as some subsequent writers have 

 suggested, by so simple a thing as the obscurity of the pebbles in 

 an unweathered block. On the contrar3% more than one of these 

 writers have described the rocks in which his statements are 

 verifiable as felsites. Such rocks may be seen near the Llanheris 

 road, on the summit of the felsite crag in the tramway section, on 

 the slopes of Clegyr, and on Mynydd-y-cilgwyn, and here certainly 

 Sir A. Eamsay's explanation is suggested, though further observa- 

 tions, here and elsewhere, render it untenable. When, however, 

 we reject it we must admit that a conglomerate can be deposited on 

 a felsite in such a way that it is impossible to say where one begins 

 and the other ends. His explanation also accounted for "the 

 capricious variation of the strata adjoining the porphyry," which 

 otherwise might have suggested to him an unconformity. 



The first attempt to upset Sir A. Eamsay's explanation came 

 from Professor Hughes and Dr. Hicks, the latter publishing his 

 views (B) in 1878.- He considered (1) the felsite at Moel Tryfaen 

 to belong to the same series as the rocks in the adit beneath the 

 hill, and (2) both to be overlain unconformably by the conglomerate ; 

 hut he gave no proof of the latter conclusion beyond a supposed 

 N.W. dip of the " metamorphic rocks," the Cambrian conglomerate 

 having, according to him, a N.E. dip. But he pointed out that the 

 conglomerates contained pebbles " appearing to resemble the rocks 

 in situ near," and stated of these conglomerates that ''here, as in 

 all other Welsh areas, they strongly define the base of the Cambrian." 



Of the rocks of Llyn Padarn he says, "it is more than probable 

 that we have in the series here some contemporaneous lavas," and 

 " the pebbles in the conglomerates are usually identical in character 

 with the rocks below." He does not, however, point out what is 

 the nature of " the rocks below," though they must have included 

 the felsite. 



In 1879 Professor Bonney published a paper on the district (C).^ 

 It gave the result of "working over parts of the district [including 

 the Bangor area] on several days in September, 1878." From 

 internal evidence it would appear that the author visited Moel 

 Tryfaen and returned by the Bettws Garmon road, walked from 

 Cwm-y-glo to Llanheris by the railway, and returned part of the 



^ Mem. Geo!. vSurvev, vol. iii. 



2 Q..T.G.S., vol. xxxiv, pp. 147-152. 



» Q.J.G.S., vol. XXXV, pp. 309-320. 



