PRE-CAMBKIAST EOCKS OP ST. DAVID'S. 309 



same zones can be recognized on each side of the promontory the 

 existence and nature of the fold are made apparent. We need not 

 look, indeed, for more than a general agreement in the repetition of 

 the volcanic part of the Section. Yolcanic accumulations are so 

 characteristically inconstant that the series on each side of the fold 

 might quite well be entirely different. The coarse tuffs and breccias 

 were doubtless thrown up in heaps round the vents from which they 

 were discharged. The lavas must have formed submarine banks or 

 reefs of but limited extent. Not only, therefore, might we expect 

 that the succession of volcanic masses on one side of the axis would 

 differ from that on the other, but there might very well be local 

 overlaps of the conglomerate upon the irregularities of the volcanic 

 masses. West of Treginnis-uchaf, where the prolongation of the 

 Ehosson diabase reaches the shore, there seems to be an instance of 

 this kind, the conglomerate ending against the diabase bank on one 

 side but reappearing on the other. Such a structure, however, is 

 obviously quite different from an unconformability. Even at the 

 locality just referred to the conglomerate is succeeded by fine 

 volcanic tuff, showing that, though the conditions of sedimentation 

 had considerably changed, volcanic action still continued. 



Even in the volcanic group, however,' some leading features are 

 repeated on either side of the fold. The Pen-maen-melyn lavas 

 reappear, though in diminished proportion, on the east side near 

 Pen-y-foel. The diabase sheet at Rhosson may be the same as that 

 of Clegyr Eoig. The Porth-lisky schists are partially exposed on 

 the coast of Ramsey Sound at Ogfeydd-duon ; and the conglomerate 

 with its overlying groups is easily traced on either side of the fold*. 



Various eruptive masses have been protruded through the stratified 

 rocks. It is possible, as I have already suggested, that some of the 

 diabase sheets in the volcanic group may be intrusive ; but if so, 

 they must still, no doubt, be classed as belonging to the volcanic 

 period, like the intrusive rocks associated with the contemporaneous 

 volcanic series in the Carboniferous system of Central Scotland. 



Of much later date are the granite and quartz-porphyries. Eor 

 reasons to be afterwards given I class these two groups of highly sili- 

 cated rocks together. A reference to the Map (PI. VIII. p. 268) will 

 show that the granite has risen through the eastern limb of the isocline, 

 considerably disturbing the symmetry of the structure. In a general 

 sense the longer axis of the granite mass corresponds with the domi- 



* The repetition of the same petrographical character has been admitted by 

 Dr. Hicks himself, as may be seen in the sections published by him in his paper 

 of May 1878 (Quart. Journ. Geol. Soc. vol. xxxiv. p. 166). In Nos. 1 and 2 of 

 these sections subdivisions 1, 2, 3, 4, 5 and 6 are described in nearly the same 

 words as subdivisions 8, 10 and 11. It is interesting to find that he makes the 

 Cambrian conglomerate to be underlain by the same succession of beds at 

 Llanhowell and Caerbwdy, though those localities are upwards of three miles 

 apart. This would hardly be likely to occur were there an unconformability 

 between the conglomerate and the rocks below it. [At the reading of this paper 

 Mr. Peach exhibited Dr. Hicks's section, coloured in accordance with what we 

 believe to be the true structure of the ground ; and he showed how entirely that 

 section is explicable on the idea of an inverted anticline.] 



Q.J.G.S. No. 155. a 



