196 H. D. Roberts — On the Twt 'Kill Conglomerate, Carnarvon. 



many instances small shifts may be detected by carefully tracing 

 bands of rock across the fissures. Many of the cases of apparent 

 passage proved on close examination to be the result of small faults 

 bringing into intimate juxtaposition the conglomerate and granitoidite. 

 The decomposition at the junctions in parts also rendered it difficult 

 at first sight to make out the relations of the rocks, but after several 

 hours' careful examination of the section at different times, I enter- 

 tain no doubt that the conglomerate is quite distinct from the 

 granitoidite and belongs to a later period. 



At the eastern end of the quarry the typical granitoid rock is 

 quarried, and it presents considerable variation in appearance in 

 different parts of the quarry ; it is traversed at one spot by bands 

 of white quartz, w^hich might well have furnished the materials of 

 the pebbles in the conglomerate. More than once in a N.E. 

 direction these beds are again exposed, notably in a quarry between 

 Tygwyn and Ysyuborwen. There the conglomerate is seen some- 

 what coarser in character than in the Twt Hill quarry, alternating 

 with beds of grit and sandstone and showing the same strike and 

 dip. Pebbles of jasper, quartzite and schist are rather more 

 numerous here and the character of the bed is well shown. 



The evidence seems therefore clearly against the view that the 

 conglomerate with its associated grits and sandstones is a part of the 

 Pre-Cambrian granitoid series. On the other hand nothing hitherto 

 said points conclusively to the base of the Cambrian as the true 

 position of these beds. To the complete and final settlement of the 

 matter it is necessary that either undoubted fossiliferous Cambrian 

 beds should be traced down into the deposits in question, or better 

 still that fossils should be found in the sandstones themselves 

 overlying the conglomerate. Both these demands can, I believe, be 

 satisfied. The evidence has been obtained in another district where 

 the exposures are more numerous and the stratigraphical relations 

 more readily made out than in Carnarvonshire. On two occasions 

 last year I had the advantage of being conducted by Prof. Hughes 

 over parts of Anglesea, where we found beds not distinguishable 

 from those exposed at Twt Hill. I shall refer to three sections. 



1. On the shore in Dulas Bay, a few miles to the S.B. of Amlwch, 

 black slates occur, in which we had the good fortune to discover 

 Graptolites. These will be described in a forthcoming paper by 

 Prof. Hughes. We traced the beds inland, and finally made a 

 traverse across the strike with the object of finding if possible the 

 basement conglomerate. The black slates we traced passing down 

 into brown sandstones, and these into a conglomerate identical in 

 petrological character with the Twt Hill conglomerate. It is well 

 shown in a quarry at Penlon, about two miles due west of Traeth 

 Dulas. The appearance of the quarry is singularly like that at 

 Twt Hill. The conglomerate, made up of quartz pebbles, rests on 

 granitoid rock and passes up into brown sandstones. 



2. Dr. Callaway states in the Geological Magazine, for March, 

 1880, p. 118, that near Nebo, two miles S.E. of Amlwch, he 

 discovered in two quarries a quartzose conglomerate, " lithologically 



