260 PROF. C. LLOYD MORGAN^ ON THE PEBIDIAN 



Carn-ar-wig point it interdigitates with more acidic lavas and 

 ashes. 



I have no wish to undervalue these facts. They are, indeed, just 

 such as I should expect upon the hypothesis that I adopt. I have 

 already mentioned the great cinder-beds in the western part of the 

 district which seemed to point to subaerial accumulation ; I have 

 also drawn attention to the tonguing-in of the conglomerate with 

 the volcanic beds at Carn-ar-wig. These facts seem to point to 

 subaerial conditions and shoals to the west. I think the facts point 

 to deeper water and more sedimentary conditions to the east and 

 south-east. But I believe that both near Pen-dal-aderyn and near 

 Pen-y-foel there is evidence of local erosion, within the volcanic 

 series, due to the fact that the tuifs were brought to or near the 

 surface, and thus suffered some abrasion from waves and currents. 

 And then, at the close of Pebidian times and the ushering in of the 

 Harlechs, the powerful current which bore onwards the materials 

 of the conglomerate could not fail to sweep away much of the 

 superficial soft tuff in the shallows and shoals of the Pebidian sea. 

 Its effect would be less marked in the deeper w^ater to the east and 

 south-east. 



Let us try to realize the conditions. Volcanic tuffs are being 

 formed around several, perhaps many, small volcanic centres, for it 

 is probable that Cambrian volcanic efforts did not concentrate 

 themselves on one focus. The volcanic centres are submarine, or 

 perhaps form islands. As in the case of Graham's Island, they may 

 have given rise to islands which were soon washed away by the 

 waves, the materials being scattered around as tuffs, commingled 

 perhaps with some sedimentar}^ materials. Then as the volcanic 

 fires are dying out (Carn-ar-wig tells us they are not quite dead), 

 there comes a powerful current sweeping over the tuffs and bearing 

 onwards rolled quartz- and quartzite-pebbles of altogether foreign 

 origin. Whence came the pebbles, how was produced the current, 

 who can say ? There is the evidence of it in the conglomerate, 

 which is no beach-deposit or anything like it. 'Now if this be 

 anything like a true story of the sequence of events, and it seems to 

 me the only view which fits the facts, is it to be expected that the 

 conglomerate everywhere should lie upon the same volcanic beds ? 

 AVould it not be in the highest degree surprising should it be found 

 to do so ? Is it not more probable that here in deeper water it 

 should rest upon more sedimentary tuffs (Ogof Golchfa) ; there, in 

 shallower water, it should be in close proximity to lava reefs (Maen 

 Bachau) ; and elsewhere (Carn-ar-wig) it should tongue in with the 

 later splutterings of the volcanic period ? So it seems to me ; and 

 thus I would account for the apparent overlap of the conglomerate 

 on the Pebidian beds. 



f. The '^ Felsitic" ByJces. — Cutting through the Pebidian strata 

 and well exposed at many points along the coast-hne are igneous 

 dykes. These are of two types, the one basic the other acidic. 



Some of the basic dykes are seen in close proximity to or actually 

 connected with the basic intrusions of Pen-y-foel and the neighbour- 



