THE CAMBRIAN SERIES LN N.W. CAERNARVONSHIRE. 331 



(c) Details of Outcrops. 



I have laid down on the six-inch map the outcrops of the rocks 

 west of the great fault, but I do not find that these afford much 

 help. As regards sundry details, Mr, Blake differs from other 

 observers, but on what grounds I fail to see. He gives no evidence 

 for reverting to the Survey mapping of a long tongue of felsite 

 running up near Friddodd *, which was removed by Prof. Bonney. 

 The only rocks that I could find in the fields near that farm are 

 slaty rocks and felsitic grits. Moreover, the great conglomerate 

 can be traced almost continuously from the shore to the eastern 

 entrance of the western tunnel, instead of being, as Mr. Blake maps 

 it, two separate masses with distinct strike. The direction of the 

 outcrop may possibly vary, or the conglomerate may have become 

 thicker in the neighbourhood of the straits. But we find it on the 

 shore, we meet with indications of it by Gored Road, and probably 

 by Hwfa Road ; we see it excavated for some distance along the 

 road east of the Look-out, and quarried at three places near new 

 houses east of the large field to the south of the road. Again we 

 find (possibly with a slight displacement eastwards) an indication 

 of it just north of, and also in the road to, Friddodd ; then it occurs 

 in the quariy behind Plas Ludwig, and is well exposed by the 

 railway at the station. Mr. Blake, it is true, is inclined to separate 

 the two masses exposed on the east and on the west of the main 

 fault respectively, because the pebbles on the east are " far more 

 quartzose." But, as it seems to me, the lithological difference is 

 not sufficiently well-marked to prove the distinctness of the two con- 

 glomerates ; for, on the east of the fault, the pebbles near the Mount 

 are mainly felsite ; and, on the west of the fault, the conglomerate 

 of the shore, although at several places almost wholly composed of 

 felstone pebbles, also contains quartzose pebbles locally. The latter 

 also are common in the road excavation, and in one at least of the 

 quarries behind the new houses. But that two different conglome- 

 rates are here faulted together is very improbable. The main 

 details in the outcrop of this conglomerate west of the fault are 

 shown in Prof. Bonney's map : as he points out, it is not easy to 

 interpret the structure of the district, but neither here nor to the 

 eastward does the new view seem to be supported by any facts of 

 importance. 



II. The Arexig Uxconformitv at Caernarvon and the Thickness 

 OF THE Ca:w^brian Series. 



All these rocks are bounded on the east by Arenig strata, which 

 Mr. Blake traces to Caernarvon, and to this epoch he refers the 

 Twt-Hill conglomerate. This is mainly an arkose of the granitoid 

 rock, from which it must therefore to a large extent have been 

 derived. Mr. Blake's hypothesis is that the granitoid was intrusive 

 into rocks which are now hidden : and that the Twt-Hill con- 

 glomerate originally rested upon the surface of these concealed 



* Quart. Journ. Geol. Sec. vol. xHv. (1888) p. 278. 



2a2 



