38 ARCHAEAN. 



The prevailing characters in the three formations of Pre- 

 Cambrian rocks made out in Pembrokeshire by Dr. Hicks are thus 

 briefly defined by him : — 



Micaceous, talcose, and chloritic schists, with 

 slaty and massive green bands containing 

 p , . ,. } epidote, serpentine, etc. 



Tuffs, indurated ashy shales, breccias, silvery 



schists, porcellanites, conglomerates, and 



green and purple agglomerates. 



. . ( Breccias, halleflintas, porphyries, and quartz- 



Arvonian . . j ^^^^.^^^^ P P ^ > 4 



iQuartzose rocks, granitoid gneiss, and compact 

 granitoid rocks, with bands of crystalline 

 limestone. 



The actual thickness of each division is stated to be "many 

 thousand feet," but it cannot be definitely calculated, although the 

 aggregate estimates exceed 18,000 feet. Older, probably, than the 

 Dimetian, according to Dr. Hicks, is the Lewisian (or Hebridcafi) 

 group, the prevailing rocks of which are massive gneisses, in 

 which hornblende and a reddish felspar are the chief ingredients. 

 They are of a dusky red, grey, or dark colour. Sometimes almost a 

 pure hornblende rock is found. The group is met with in parts 

 of the IVIalvern chain. 



The Dimdian series, which includes rocks coloured as Granite 

 or Syenite on the Geological Survey Maps near St. Davids, com- 

 prises rocks of a granitoid character, usually of a massive, but 

 sometimes of a schistose nature. The rock is largely made up 

 of quartz, with some pinkish or white felspar. The upper portion 

 {Porlhlisky group) comprises the Lower Moor rock and the highly 

 quartzose rocks of Porthlisky and Porthclais ; the lower division 

 {B}y7i-y-garn group) includes the massive granitoid rocks of Bryn- 

 y-garn, and Brawdy, St. Davids. A peculiar brecciated bed in the 

 Dimetian of St. Davids has been produced in place by weathering 

 along the joint planes.' 



The Arvonian series consists mainly of flows of rhyolitic lavas, alter- 

 nating with felsitic breccias and halleflintas. These were formerly 

 grouped as intrusive felstones, porphyries, etc. The series is 

 largely developed in Pembrokeshire. Dr. Hicks remarks that the 

 junction with the Dimetian is abrupt, and may be seen at St. 

 Davids, about a quarter of a mile to the south of the Cathedral, 

 and near Rock House. Excellent sections of Arvonian rocks may 

 be seen in the gorge through which the western Cleddau river 

 flows, and through which the road passes from Haverfordwest to 

 Fishguard. Two divisions are made, the Upper {^Holyhead group) 

 comprising the schistose and compact quartz rocks of Llanhowell, 

 near St. Davids, and of Holyhead mountain; the Lower {Treffgarn 



» T. McK. Hughes, G. Mag. 18S3, p. 306. 



