1 60 H. HICKS ON THE DmETlAN AND 



beach-pebbles on an old coast line, the cliffs of which these rocks 

 once formed, before they were submerged to receive the succeeding 

 Cambrian sediments. 



In Section III. (fig. 3) another patch of these Pebidian rocks is 

 seen. This is on the coast near Newgale, about eight miles east of 

 St. David's. The beds here, as in the other areas, are nearly ver- 

 tical, and are chiefly felstones of a flesh- colour, with specks of horn- 

 blende. These are again flanked by Cambrian conglomerates con- 

 taining pebbles identical with the rocks below. To the north-east 

 of this mass is seen the second Dimetian axis, which extends for 

 about six miles in an E.N.E. direction. 



Between this axis and the one at St. David's a broken succession 

 in several anticlinals and synclinals of Cambrian rocks is found, the 

 highest being Tremadoc rocks in a synclinal curve near the centre, 

 the lateral extensions of the older rocks forming therefore a kind of 

 basin into which these have been compsedres. 



From the foregoing descriptions it will be gathered that a very 

 large proportion at least of the Pebidian rocks must have had a 

 volcanic origin. As, however, they were at first subaerial and 

 afterwards submarine accumulations, it is evident that they must 

 also be partly detrital. The lowest rocks are the agglomerates ; 

 and in these I think we have clear evidence of proximity to a sub- 

 aerial volcano surrounded by the ordinary materials of a volcanic 

 cone, the largest proportion of the masses being fragments of lava 

 which evidently had cooled under atmospheric influences. Again, 

 associated with these fragments we find angular bits of chloritic 

 schists, ashy shales, and quartzites, similar in every respect to 

 beds now visible in the Dimetian series below. These bits may 

 have been broken off in the eruption, or they may be fragments 

 caught up by the cementing lava as it flowed over the land. The 

 next sediments indicate the undoubted submergence of the area, as 

 the masses are partially rounded and stratification is visible. These 

 are followed by beds of fine sediments, and subsequently by a series 

 of well-bedded ashes and lavas, which had evidently been cast out 

 in a sea over a gradually subsiding area. The general likeness in 

 the whole series to groups of volcanic rocks found in more recent 

 strata is most marked, especially to those associated with the Lower 

 Silurian in many areas : and, with the exception perhaps of a 

 higher stage of metamorphism, there would be but little to guide 

 one to an understanding of their age, if exposed in areas where the 

 succeeding sediments had been removed. This may be one of the 

 reasons why these rocks have not been more frequently recognized 

 hitherto, especially as, until very recently, we were unaware of the 

 occurrence of volcanic rocks of such great antiquity which retained 

 any thing like so marked a character as these rocks have done *. 



* It is an interesting fact, and one likely to prove useful as a future means 

 for the correlation of these rocks with those in other areas, that similar changes 

 seem to have taken place over considerable portions of the northern hemi- 

 sphere, especially those bordering the Atlantic, at this epoch in geological time. 

 According to the descriptions by Mr. Matthew, Quart. Journ. Geol. Soc. vol. 



