242 PROF. C. LLOYD MOEGAN ON THE PEBIDIAN 



up to US in the rocks of St. Davids of age behind age most pleasing 

 to the imagination of the geologist." 



The second interpretation is that advocated by Dr. A. Geikie 

 (Quart. Journ. Geol. Soc. vol. xxxix. p. 261). According to this 

 view the Pebidian constitutes a volcanic series at the base of and 

 forming an integral part of the Cambrian s3'Stem ; the Dimetian 

 consists of granite intrusive into the Cambrian and of much more 

 recent age ; while the Arvonian group comprises portions of the 

 Pebidian volcanic breccias and tuffs, where these are invaded by 

 quartz-porphyries associated with the intrusive Dimetian granite. 



To these two divergent interpretations Mr. J. P. Blake (Quart. 

 Journ. Geol. Soc. vol. xl. p. 294) has added a third, first suggested 

 by Mr. Hudleston. He contends that the Dimetian (granite), 

 Arvonian, and Pebidian belong to one volcanic series, the age of 

 which is anterior to and independent of the Cambrian epoch. 



Not only is there this marked divergence of opinion as to the age 

 of the Pebidian volcanic series and its relations to the other rock- 

 masses of the district, but equally marked is the divergence of 

 opinion as to the succession of the Pebidian strata and the mode of 

 their stratigraphical arrangement. Dr. Hicks maps the Dimetian- 

 Arvonian ridge as the ancient land-axis against which the Pebidians 

 were unconformably deposited and from which they now dip away 

 to the north and to the south. The series to the north of the axis 

 is thus a continuous ascending series, the older beds, except where 

 they are " dropped" by faults, flanking the ridge, the newest being 

 those furthest from the axis found near Whitesand Baj^ to the north 

 of St. Davids. Dr. Geikie, on the other hand, denies that the series 

 to the north is a continuous series. He contends that they are 

 repeated by an isoclinal fold " the axis of which must cut the coast- 

 line somewhere between Pen-maen-melyn and Pen-y-foel." The 

 beds to the south of the axis of this isocline are thus, on his view, 

 repetitions of those to the north, and their northerly dips are 

 reversed dips. Mr. Blake, however, examined the coast between 

 Pen-maen-melyn and Pen-y-foei, and though he was " enchanted 

 with the glorious confusion of the volcanic masses there exhibited," 

 could find no sign of a regular fold, " which," he says, " among such 

 rocks would be almost impossible to conceive." 



It can hardly be said that our knowledge of the Pebidian volcanic 

 series of St. Davids has yet reached a satisfactory position of stable 

 equilibrium. 



I have recently spent some seven weeks in the St. David's 

 district, nearly the whole of which time I devoted to field-work *. 

 I have examined every accessible portion of the cliffs and all the 

 inland exposures, having the advantage of the 6-inch Survey map 

 on which to record my observations. The discovery of some new 

 facts and the formation of an independent opinion on some of the 

 debated questions impels me, though not without diffidence, to enter 

 the field of the St. David's controversy. 



* During the greater part of the time I had the advantage of the assistance 

 and companionship of Mr. J. G. Grenlell, M.A., F.G.S. 



