168 H. HICKS ON THE DI11ETIAN AND 



character in the Dimetians, great masses of quartzy beds being 

 mixed up with granitoid rocks and peculiar quartz-porphyries, 

 such as that described by Mr. Tawney, the whole forming a crys- 

 talline acidic series, yet pervaded in many places by dark-green 

 basic rocks, which, there is good reason to believe, are old dolerites. 

 A very similar sort of rock occurs in the quartz -felsite mass of Llyn 

 Padarn. The Pebidian rocks are more volcanic in their character, 

 and, although in so large a series there must be great variety, the 

 lower beds clearly contain abundant debris of acidic erupted rocks, 

 such as the ancient rhyolite so well described by Mr. Davies. The 

 quartz-felsites of Llyn Padarn, which are held by the authors to. 

 occupy an analogous position in the pre-Cambrian series of North 

 Wales, fairly tally in petrological character, and may represent the 

 acidic volcanic series of that area. The Upper Pebidian, however, 

 as shown in the diagram, contains a group of rocks which have no 

 representative elsewhere. The peculiar rock of Clegyr-foia may be 

 taken as a type of these. They are in all probability bedded lavas 

 and ashes of a basic character, which, on that account, have under- 

 gone more chemical alteration, and are well worthy of close study. 



Taking all the facts into consideration, it is not unreasonable to 

 suppose that in the more crystalline lower series, as developed near 

 Caernarvon and St. Davids, we see the hypogene conditions of a 

 great mass of rocks, whilst the beds between these crystalline 

 masses and the great Cambrian conglomerate represent a more or 

 less contemporaneous outpouring of lavas and ashes, partly sub- 

 aerial, but, as regards the Pebidian especially, in a great measure 

 remodelled and deposited by water. 



Prof. Seeley remarked that as the whole argument rested on the 

 examination of microscopic slides, upon which it was impossible to 

 form a judgment from a hasty inspection, he did not see how the 

 results arrived at by the authors could be satisfactorily discussed. 

 He inquired, however, whether the succession of the beds as de- 

 scribed by Mr. Hicks was such as to preclude those noted as lavas 

 and ashes being really of metamorphic origin. 



Mr. Drew said that even a cursory examination seemed to prove 

 that some at least of the specimens were undoubtedly derived from 

 ash -beds. 



Mr. George Maw, in taking exception to the view expressed by 

 Prof. Hughes, that the break he had pointed out in 1867 as included 

 in the section near Llanberis, in the cutting of the Caernarvon and 

 Llanberis Railway, was not due to unconformity, wished to know 

 at which point below the horizon of this section Prof. Hughes had 

 recognized unconformity between the Cambrian series and under- 

 lying porphyritic masses between Llanberis and Caernarvon. 



Mr. Hicks said, in reply to Prof. Seeley, that there could be no 

 doubt that a very large proportion of the Pebidian rocks were made 

 up of volcanic materials stratified ; but he quite believed that there 

 was also a great amount of ordinary detrital sediment mixed up 

 and interstratified with the volcanic materials, as evidenced by the 

 presence of clay slate, hornstones, and porcellanites, &c. 



