TEANSACTI0N8 OF SISCTION C. 



Trefyarn.— The pre-Cambrian rocks in this area are mainly felstones of a 

 peculiar type and volcanic ash. The conglomerates which repose on these rocks 

 contain pebbles of large size, and have been proved by microscopical examination 

 to be identical in character with the rocks on whose eroded surface they repose. 

 Here again the marked similarity in the minutest particulars between the rolled 

 fragments and the underlying rocks proves indisputably that the peculiar changes 

 which these rocks have undergone must have taken place before the fragments were 

 broken off, therefore in pre-Cambrian times. 



Meeionethshieb. 



Harlech Mountam.—Nea.T the centre of the well-known anticlinal of Cambrian 

 rocks in the Harlech Mountain there are conglomerates exposed which contain 

 fragments of granitoid rocks, felstones, &e., in addition to pebbles of quartzites 

 and quartz, and it is clear that they are, though not actually at the base, yet very 

 near the base of the Cambrian rocks of that area. The most important con- 

 glomerates, however, in this district are those which were discovered by Professor 

 Hughes and the author on the east side of the Trawsfynnydd Road, between Cae 

 Cochion and Penmaen. Here they are seen resting unconformably upon an older 

 series of rocks, and large fragments of the latter occur plentifully in the con- 

 glomerates. 



Akgleset, and Caeenaevonshiee. 



As Sir A. Geikie has recently admitted that many of the rocks in Anglesey, 

 coloured on the Geological Survey Map of that island as 'altered Cambrian (aiid 

 partly Silurian),' are ' undoubtedly far older than at least any of the Cambrian 

 rocks of Anglesey or Carnarvonshire,' the evidence furnished by the basal beds 

 where they rest on these rocks is highly important. The author was the first to 

 point out, in a paper read before the British Association in 1879, that the patch 

 near the centre of Anglesey coloured as ' intrusive granite chiefly of Lower Silurian 

 age ' contained within its boundary rocks of pre-Cambrian age, and evidently the 

 oldest rocks in the island. (The rocks in this patch Sir A. Geikie now says appear 

 to him to be ' unquestionably Archaean.') In the year 1884 the author further 

 showed that the Cambrian conglomerates near Llanfaelog contained large pebbles 

 of granitoid and other rocks, which, on microscopical examination, proved to be 

 identical with rocks m situ in their immediate neighbourhood. 



Professor Hughes has shown by fossil evidence that the beds which overlie the 

 conglomerates near Llanerchymedd are of Upper Cambrian age, and, as these are 

 separated by faults from the conglomerates and gi-its, it is clearly justitiable to 

 classify these beds as the basal beds of the Cambrian in that area. The basal 

 Cambrian beds near Beaumaris furnish equally convincing proofs of proximity to a 

 shore-line composed of pre-Cambrian schists and felsitic rocks. 



Bangor and Caernarvon. — The basal beds at and near Caernarvon described by 

 Professor Hughes show clearly that they must have been deposited along a shore- 

 line where granitoid and felsitic rocks were undergoing denudation, and the absence 

 there of the usual thickness of overlying Cambrian roclcs is due, the author believes, 

 mainly to faults, but in part also to the unevenness of the pre-Cambrian land sur- 

 face. There is much evidence in the various areas to show that the pre-Cambrian 

 land surface was very irregular in character, and that the Cambrian sediments 

 were accumulated along fairly well-defined lines of depression. 



Bethesda, Llyn Padarn, and Moel Tri/faen.~The basal beds of the Cambrian in 

 these areas, where not removed by faults, are also conglomerates, and the frag- 

 ments in the conglomerates are mainly such as would be derived by denudation 

 from the ridge of rocks in the neighbourhood which had been claimed by the 

 author and Professor Hughes as of pre-Cambrian age. These views, put forward 

 in the year 1877, were not accepted by the chiefs of the Geological Survey ; but 

 m the year 1891, in his anniversary address to the Geological Society, Sir A. 

 Geikie admitted that the rocks in this ridge, ' variously termed quartz porphyries, 

 felsites, and rhyolites,' were not intrusive in the Cambrian rocks, but ' tho oldest 



