ARCH/EAN. 39 



group) comprising the breccias, halleflintas and quartz felsites of 

 Treffgarn Mountain and Roch Castle. The " Treffgarn Rocks " 

 are prominent weathered masses of these rocks. 



The halleflintas were first recognized by Mr. T. Davies (when 

 examining some slides for Dr. Hicks in 1878) as being of the 

 peculiar nature characteristic of the rocks so well known under 

 that name in Sweden. The term, says Dr. Hicks, is used generally 

 for a compact rather flinty-looking rock, which, in chemical com- 

 position, does not diff"er much from one of the more acid varieties 

 of rhyolites, and may be either one of these considerably changed, 

 or an altered sedimentary rock. Mr. Davies regards the halleflintas 

 as quite distinct from the felstones, as the former contain a larger 

 proportion of silica, and are of very diff"erent physical character. 



The Pebidian group is less altered in character than the Arvonian 

 or Dimetian. According to Dr. Hicks, agglomerates and breccias 

 occur in great thickness 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 occasionally purple and green slates. Serpentinous 

 bands are also sometimes 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. Two divisions 

 are made, the Upper {^Trcginnis group) comprising the conglome- 

 rates, breccias, schists, and slates, with contemporaneous lava- 

 flows and volcanic ashes, and with limestones and serpentines; and 

 the Lower {Menai group) comprising breccias, chloritic, talcose, 

 felspathic and micaceous schistose rocks, also limestones and 

 serpentines. This description of the divisions proposed by Dr. 

 Hicks gives the general characters of the beds recognized in other 

 parts of the country, as well as in Pembrokeshire. The Treginnis 

 group is exposed at Treginnis, on the shore of Ramsey Sound, 

 near St. Davids. 



The conglomeratic beds at the base of the Pebidian in Pembroke- 

 shire are largely made up of masses derived from the Arvonian 

 rocks ; and Dr. Hicks states that an actual unconformity between 

 the two groups may also be seen at several points. The Cambrian 

 strata rest unconformably upon them.^ 



The conclusions thus made known by Dr. Hicks have, however, 

 been contested by Dr. A. Geikie. Referring first to the gneissic 

 or granitoid rock of Bryn-y-garn, described by Dr. Hicks as a 

 bedded metamorphic rock, Dr. Geikie points out that he found no 

 evidence of foliation ; that whether in mass, in hand-specimens, or 

 under the microscope, it presents the ordinary structure of granite. 

 And he observes that at St. Davids, the coast-section and the trans- 

 verse valley cut by the river Allan permit the actual junction of the 

 granite with the surrounding rocks to be seen at several localities. 



' Pop. Science Review, 18S1, p. 289; Proc. Geol. Assoc, vii. 63; and G. 

 Mag. 1879, p. 435. 



