THE CAMBRIAN SERIES IN N.W. CAERNARVONSHIRE. 333 



of drift, or they may be represented by beds which are less strongly 

 cleaved *. 



Mr. Blake speaks as if doubtful of the occurrence of the Dinas- 

 Dinorwic conglomerate, noted by Dr. Hicks ; but I found this rock 

 clearly exposed at several places, with pebbles of felsite more than 

 one inch in length (two inches and even seven inches). As boulders 

 are scattered over the hill, it is indeed possible that the masses 

 exposed are not in situ, but, for erratics, thoj are of very consider- 

 able size. For instance, several blocks occur along a scarp on the 

 hillside near Pen-y-groes, two of which are quite eight feet in 

 length. But even if this conglomerate is not in situ, why has 

 Mr. Blake omitted from his section f others which undoubtedly 

 occur, like the well-marked felsitic conglomerate east and south- 

 east of Bryn Madog J, and one outcropping in a field to the east 

 of Pont Rhythell § ? 



(b) Value of the Conglomerate as a Base. 



Mr. Blake's hypothesis does not admit the basal character of the 

 conglomerate ; it is, however, a rock very largely formed of 

 materials from the underlying felsite, and it is difficult to under- 

 stand how such an extensive denudation of the igneous rock could 

 occur in the midst of a continuous succession. lb seems rather 

 inconsistent to assume that an important break occurs beneath the 

 conglomerate of Caernarvon, while here no such inference is made. 

 Moreover, in theMoel-Tryfaen exposure, Mr. Blake argues that certain 

 of the slaty pebbles are derived from Cambrian rocks "further up the 

 series '" than the beds near Bangor. But, without a long interval, 

 it is difficult to account for the induration and mineralogical change 

 which have occurred in the material of the fragments. We should 

 have to believe that, at some epoch after the deposition of one of 

 Mr. Blake's successive conglomerates, the slates of which we now 

 speak were deposited, indurated, modified and worn down to form 

 some of the Moel-Tryfaen pebbles — a process of rapid manufacture 

 indeed ! 



It is no doubt difficult to prove an unconformity below the con- 

 glomerate ; indeed, it may even be locally absent. For instance, in 

 the section to the east of Llyn Padarn I fail to find the clear proof 

 of it which Prof. Green describes. The strongest evidence in his 

 opinion was the vertical direction of the laminae in bed A. These, 

 however, had a suspicious aspect, and proved on microscopic exami- 

 nation to be igneous rock. They are thus dykes of diabase, which 

 owe their schistosity to pressure. It is no doubt curious that so 

 many should occur (eight at least in an area of a few square feet), 



* See * Geol. of North Wales,' 2nd ed. \). 185, note. 



t Quart. Journ. Geol. Soc. vol. xliv. (1888) p. 287. 



\ In tbe map (/. c. p. 272) Mr. Blake has marked a band of conglomerate, 

 but has indicated it as quartzose. Of the pebbles which I noted near Bryn 

 Madog those of felsite were in a majority, although a fair number were of 

 quartzite. In the section (p. 287) the conglomerate is entirely omitted, as 

 stated above, only a coarse grit (3) being shown. 



§ See also reference in Survey Memoir to such outcrops ; ' G^ol. of North 

 Wales/ 2nd ed. p. 185. 



