260 EEV. J. p. BLAKE ON THE KOCKS MAPPED AS 



Pachell occiipying the same position, below the curious muddy con- 

 glomerate with red pebbles, and it is here only, if ever, that we can 

 reach the base of the Cambrian in Caernarvonshire. There is indeed 

 a small-pebbled conglomerate in the Felin Hen area to the west of 

 the Laminated Grits, which might be the same, but it might also be 

 some other conglomerate developed in the mass. There is certainly no 

 evidence that it lies here upon felsite ; it probably does not. 



Such is the Cambrian succession in Caernarvons^hire as I read it. 

 It is interesting to trace how the early volcanic outbursts of the 

 period which gave rise to the felsites of Dinorwig were followed by 

 the deposits of rough and rapid waters capable of rounding the hard 

 rocks, and of carrying tliem miles from their source ; how the alter- 

 nations of condition, tidal or otherwise, gave rise first to the 

 Laminated, then to the Banded Slates and Grits ; how some new 

 outburst provided so manj^ fragments of the Monian slates that 

 the waters nearest Bangor could scarcely round or disintegrate them, 

 soft as they may have been ; how a still later lava-outbu rst provided 

 a fresh source of huge pebbles and gave apparently fresh powers to 

 the waters to roll and remove them ; and how finally tranquillity 

 set in, and except for the occasional intrusion of grit-bearing rivers 

 the sea set steadily to work to disintegrate, transport, and deposit 

 the materials for the vast masses of our fine-grained roofing slates. 



Apart from the idea thus afforded us of the conditions that pre- 

 vailed during the Cambrian period — which is after all the main object 

 of our researches — the description just given shows as plainly as 

 possible that there is nothing in the least peculiar about the Bangor 

 district : the rocks are not altered there more than anywhere else, 

 and they correspond bed for bed with the rest of the Carabriaa 

 series. Indeed, with the exception of the upper parts of the Purple 

 Slate and the associated grits, they show as complei^e a sequence as 

 any other locality that may be pointed out. There is, therefore, 

 not the slightest ground for seeing anything pre-Cambrian there, 

 and even my own supposition that they were the lower parts of 

 the Cambrian series, elsewhere unseen, is proved to be erroneous. 



But a question naturally arises from our review of the conditions 

 of sedimentation, namely as to where was the centre of the volcanic 

 eruptions whose products we find embedded in the slate. With 

 regard to the Tairfi'ynnon Conglomerate, we may imagine that 

 the old Dinorwig centres were not yet exhausted, though we cannot 

 point very definitely to any fresh evidence of eruption there. 



The Bangor Breccias must have had their origin in an explosion 

 unaccompanied by lavas, and hence we should scarcely expect to find 

 any relics of the orifice. Bat in the case of the Bangor Conglomerates 

 and their equivalents to the south, lying as they do in the midst of 

 the banded series, we ought to be able to point to some fresh source 

 of the felsite-pebbles. One such source is ready to our hand, — the 

 Llyn Padarn felsite, whic]? we might thus expect a priori, to be 

 a mid-Cambrian outburst, and as such, independently of controversy, 

 to be worthy of particular study. 



