ANNIVERSARY ADDRESS OF THE PRESIDENT.' 



57 



N.W. edge. Tho texture of this new group is hard and flinty, 

 some portions having an imperfect conchoidal fracture. Tho chief 

 character or peculiarity consists in the manner in which the quartz 

 is separated or segregated into nests, giving the rock a pseudopor- 

 phyritic appearance. 



The Arvonians, both generally and microscopically, closely re- 

 semble the Swedish " Hiiileflintas ; " nevertheless associated with 

 them are true quartz-felsites, probably old lava-flows (rhyolitic). 

 The evidence as to the position which the Arvonian occupies in the 

 Pre-Cambrian group is conclusive, as determined by the way^ in 

 which the Cambrian rocks are faulted down against it. It is cer- 

 tainly Pre-Pebidian, as fragments of the Halleflinta occur in the 

 overlying Pebidian conglomerates ; its rocks occupy a large pro- 

 portion of the Dimetian axis of St. David's ; and, according to the 

 researches of Hicks, that part coloured as syenite and felstone in 

 the Geological map to the N.E. of St. David's must be assigned to 

 the "Arvonian" group, only the lower portion being Dimetian. 



Like the Dimetian these rocks are greatly altered, consisting of 

 highly metamorphosed indurated porcellanitic shales. The beds 

 resting upon the Dimetian axis are hard compact conglomerates, 

 composed of quartz and altered shale derived from the Dimetian, 



Pebidian*. — The Pebidian rocks are unconformable to the Dime- 

 tian, extending along both sides of the Dimetian ridge. They 

 strike from S.W. to N.E., nearly parallel to the ridge, or in 

 accordance with the overlying Cambrian Rocks ; they are irregular 

 in thickness and greatly metamorphosed, composed of porcellanitic 

 shales; "the narrow dark lines" of stratification and closely 

 approximating and intersecting joints distinguish the Pebidians 

 from the Dimetian, against which they rest. The " lower beds are 

 hard compact conglomerates, composed of masses of quartz and 

 altered shales, " derived from the underlying rocks and closely con- 

 nected together. These all immediately lie on the Dimetian axis. 

 They are to be seen at Nun's Well, south of St. David's, on both 

 sides of the Caerbuddy valley to the east of the city, and north and 

 south of the cathedral in the valley. This Pebidian group supports 

 the true Cambrian rocks at many places in the neighbourhood. 

 Little more than 3000 feet are exposed ; and the strike is nearly 

 identical with that of the overlying Cambrian. The large area 

 coloured as intrusive greenstone upon the Geological Survey map, and 

 extending parallel to Pamsey Sound, is composed of the Pebidians. 

 A considerable portion of the S.W. part of Parnsey Island consists of 

 the compact porcellanites which characterize this series. The 

 Harlech conglomerates rest unconformably upon or along the N.E. 

 edge of the Pebidiansf. Basic lavas and breccias predominate over 

 the Phyolites. 



It is stated by Professor Pamsay, in the ' Geology of North Wales ' 



* So named from " Pebidiauc," the name of the division or hundred where 

 these rocks are exposed near St. David's. 



t See Dr. Hicks, Quart. Journ. Geol. Soc. vol. xxxiii. pp. 229--23D, for map 

 and sections &c. 



