VOLCANIC SERIES OF ST. DAVIDS. 265 



is " diabase " subsequently intruded, it is to me inconceivable that 

 this alone should have produced the large amount of alteration and 

 induration in the conglomerate and large masses of Pebidian strata. 

 I am aware that Prof. Bonney says that he has seen nothing to 

 lead him to suspect that the Dimetian was intrusive in the Cambrian, 

 but much to induce him to consider the Dimetian far the older. I 

 can only say that oft-repeated study of the locality left me unable 

 to account for the phenomena there presented, by either faulted 

 crushing or the influence of the " diabase "-dykes. The view that 

 the Dimetian was intrusive was therefore forced upon me. 



f. General Considerations. — I have already stated that my study 

 of the Pebidian strata at St. Davids led me to dissent as widely 

 from Dr. Hicks's classification and mapping of the volcanic series 

 as from his conclusions as to their Archaean age. Quite apart from 

 any Dimetian question, I am led to regard the Pebidian beds in 

 St. Non's Bay and in Porthlisky Bay as very near the top of the 

 series. This being so, it is quite impossible to map the Dimetian 

 on the hypothesis of its Archaean age without so extravagant a use 

 of faulting as to stagger any sober geological surveyor. In any 

 case, even granting the correctness of Dr. Hicks's Pebidian con- 

 clusions, it is exceedingly difficult to realize the sequence of events 

 which could possibly have given rise to the state of things depicted 

 in Dr. Hicks's map. 



I venture to think that Dr. Hicks hmself, and some of those 

 who accept his reading of the St. David's geology, have failed to 

 realize what his map commits them to. At Ogof Golchfa, for example, 

 the conglomerate is stated to rest unconformably on the highest 

 visible beds of the Pebidian. These are not necessarily the highest 

 existing beds. Other higher beds may, on this view, have been 

 overlapped by the basal conglomerate of the Cambrian. In any 

 case, however, these Ogof Golchfa Pebidians are, according to 

 Dr. Hicks, some 8000 feet above the base of the system, which may 

 have reposed directly on Dimetian, or may have rested on intervening 

 Arvonian strata. Let us say that it rested on Dimetian, and that 

 8000 feet represents the thickness of the Pebidians. Now west 

 of St. JN^on's Bay the Cambrian conglomerate rests (see Dr. Hicks's 

 map) directly on Dimetian — that is to say, it has overlapped some 

 8000 feet of upturned strata. Here again I do not press the 

 criticism that in St. Kon's Bay the dip and strike of Pebidian and 

 Cambrian are practically identical — a state of things hard to realize 

 in connexion with such an overlap. What I wish to draw atten- 

 tion to is that such an overlap involves an enormous break in 

 time, — uplift, folding, and vast amount of denudation — between 

 Pebidian and Cambrian. And yet Dr. Hicks remarks in his reply 

 to the Director-General : " Whether the volcanic fires had com- 

 pletely expended themselves or not before the conglomerates were 

 deposited may be an interesting point ; but it is virtually of no 

 importance in regard to the questions at issue " ! and Professor 

 Bonney says, in a letter quoted by Dr. Hicks (U. J. G. S. xl. p. 546), 

 that " we do not efface the limits between Miocene and Pliocene 



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