266 PROF. C. LLOYD MOIiGAiV ON THE TEBIDIAN 



because, in a country where there has been great volcanic activity in 

 the former, some feeble sputterings have occurred in the latter " — the 

 implication being that there might be (though Dr. Eonney is not 

 satisfied with the evidence of the fact), still there might be some 

 sputterings of volcanic activity continued from Pebidian into 

 Cambrian without being utterly and completely subversive of 

 Dr. Hicks's Precambrian theory. 



Even on Dr. Hicks's own hj'pothesis, then, his mapping of the 

 district, and especially of the Dimetian with its nine-faulted boun- 

 dary, is, I will not say an impossibility, but, to me, an incomprehen- 

 sibility. 



Put great as is the difficulty of majDping the Dimetian as Precam- 

 brian, on Dr. Hicks's view of the Pebidian succession and orderly 

 sequence, it becomes immensely greater if the existence of an isocline 

 come to be regarded as an established fact. A glance at the diagram- 

 section (fig. 5, p. 257) will show how considerable a fault thei^e must 

 be between Dimetian and Pebidian on the hypothesis of the Pre- 

 Pebidian age of the former. I do not say that the Pre-Pebidian 

 interpretation is thereby rendered impossible, but I do contend 

 that the difficulties of this view are thus rendered exceedingly 

 great. 



g. The Evidence from induded Frar/ments. — On this head I have 

 nothing but negative evidence to offer. In Porthlisky Bay there 

 are beautifully rounded pebbles of Dimetian, readily recognizable 

 as such. Not a single pebble in any respect resembling these have 

 I seen in the conglomerate or elsewhere. 



Dr. Hicks lays a good deal of stress on the minute pebbles, 

 supposed to be Dimetian, in the grit-beds at Chanter's Seat. Xow 

 from Dr. Hicks's own statement and from my own observations I 

 conclude that these Chanter's- Seat beds are from 800 to 850 feet 

 above the conglomerate. I find it very difficult to conceive, on 

 Dr. Hicks's hypothesis, that any Dimetian — worn down as it had 

 been by Pre-Arvonian, Pre-Pebidiau, and Pre-Cambrian denuda- 

 tion — could have continued still to afford materials for Cambrian sedi- 

 mentaries after 800 feet of these strata had already been accu- 

 mulated. If the Dimetian origin of these pebbles be thus rendered 

 improbable, we must search elsewhere for the source of the " dirty 

 quartz." 



5. CoNCLTJSTOlSr. 



It only remains for me to state briefly the general conclusions 

 that I have reached. 



The Pebidian beds constitute a volcanic series, the base of which 

 is nowhere seen, about 1200 to 1500 feet in thickness. Their age 

 is anterior to the Harlech (Caerfai) conglomerate, which is, in 

 many places, locally unconformable to them. I advocate the reten- 

 tion of the term Pebidian either as a sub-group of the Cambrian co- 

 ordinate with Harlech, Menevian, &c., or as a distinct volcanic group 

 coordinate with Cambrian, Ordovician, &c. (preferably the former), 



