PRE-CAMBRIAN ROCKS OF ST. DAVir/s. 283 



the Church Schools forming part of that group, also on the south 

 side of the ridge near Nun's Chapel. 



So far, therefore, as regards the evidence to be obtained at St. 

 David's itself, there is no foundation whatever for the institution of a 

 separate group under the name of " Arvonian." The rocks so called 

 by Dr. Hicks are portions of his " Pebidian " group invaded and 

 altered by a central core of granite and abundant dykes or bosses of 

 quartz porphyry*. What the " Pebidian " group really is must now 

 be considered. 



3. " Pebidian." 



The general characters of this group of rocks at St. David's are 

 thus summarized by Dr. Hicks : — 



. " Most of the rocks in this group differ from those already de- 

 scribed, though occasionally there is a certain amount of resemblance 

 remaining. Instead of the acid types prevailing, as in the pre- 

 viously named groups, we find the basic types more largely developed. 

 Basic lavas and breccias now predominate over the rhyolites ; and the 

 clastic rocks are more micaceous, chloritic, and talcose. On the Survey 

 Maps these rocks are coloured as altered Cambrian, and partially as 

 intrusive greenstones. On more careful examination the so-called 

 greenstones turn out to be bands of indurated volcanic ashes, and con- 

 temporaneous basic lava-flows. Agglomerates and breccias occur in 

 great thicknesses in the group ; and the fragments are chiefly, except 

 in the lowest beds, of a basic character. Chloritic, talcose, felspathic 

 and micaceous schistose rocks occur also at various horizons, and occa- 

 sionally purple and green slates. Serpentinous bands are also some- 

 times found, as well as veins of jasper, epidote, and asbestos. Some 

 of the finer and more quartzose beds assume a gneissose appearance, 

 and others are porcellanitic. 



" The strike in this group is from about N.E. to S.W., and hence 

 nearly in accordance with that in the overlying Cambrian rocks. 

 That this group, however, must have been in much the same con- 

 dition in which it is found, before a grain of the Cambrian rocks 

 was deposited, is perfectly clear from the fact that the conglomerates 

 at the base of the latter are very largely made up of rolled pebbles 

 and rounded fragments identical with the rocks below. An actual 

 unconformity between the two groups is also seen at several points. 



" This group consists of a far more varied series than the two 

 former, and doubtless would exhibit a still greater diversity if fully 

 exposed ; for it is perfectly clear that, in consequence of the rapid 



* The intrusive character of the quartz porphyry south of Nun's Chapel is 

 admitted by Dr. Hicks in his paper of 1877 (Quart. Journ. Geol. Soc. vol. xxxiii. 

 p. 236). He gives a section showing it cutting through the rocks (his " Pebidian"), 

 and says that it does not penetrate the Cambrian beds above. But at its western 

 end, where it descends to the beach, it approaches the conglomerate, and would 

 probably be found piercing it if the beach could be cleared of the fallen d6bris. 

 I shall show that this rock is precisely similar in petrographical character to the 

 so-called " Arvonian " porphyries of St. David's (p. 315). 



