150 H. HICKS OK SOME PKE-CAMBEIAN (DIMETIAK 



this higher state of alteration may have been induced in them in 

 consequence. It is more than probable also that we have in the 

 series here some contemporaneous lavas, and that the compact 

 felstone weathering white on the surface so as to give a brecciated 

 appearance, to be seen to the north-west of Llyn Padarn, is chiefly 

 of this nature, though also in part a true breccia. The Cambrian 

 conglomerates are frequently found resting on the central so-called 

 porphyritic series along both sides ; and the pebbles in the conglo- 

 merates are usually identical in character with the rocks below. 



These are usually distinctly rounded, and generally imbedded in ' 

 either an unaltered or a semicrystalline matrix, from which they } 

 can be easily removed. They were evidently in their present state, as 

 regards consolidation, before they were cemented together to form 

 the conglomerate, and must have been derived from rocks already 

 highly metamorphosed at that time — such rocks, indeed, as now occur 

 immediately under them, and which, we venture to believe, belong to 

 a Pre-Cambrian series. 



Sometimes these conglomerates strike up against the central 

 series ; but when this occurs it is found that they are brought into 

 that position by faults, the far more usual positions being the 

 flanking of the series on both sides. 



It has been supposed by Prof. Ramsay and others that these 

 conglomerates, especially at the eastern edge of the porphyritic se- 

 ries on the south of Llyn Padarn, are altered by and actually pass 

 into the so-called porphyry. 



The following description from Prof. Ramsay's memoir on 

 North "Wales will explain his view : — " So closely does the matrix of 

 the altered rocks resemble the adjoining typical porphyry in colour, 

 texture, and even in porphyritic character, and by such insensible 

 gradations do they melt into each other, that the suspicion or, rather, 

 the conviction constantly recurs to the mind that the solid porphyry 

 itself is nothing but the result of the alteration of the stratified 

 masses carried a stage further into the region of that kind of ab- 

 solute fusion that in so many regions resulted in the formation of 

 granites, syenites and other rocks, commonly called intrusive." This 

 alteration, he believes, took place towards the close of the Silurian 

 period. 



The view held by Prof. Eamsay is also, I find, supported by Mr. 

 G. Maw, in his paper in the Geological Magazine, vol. v. No. 3 ; 

 and the unconformity supposed by him to be present here is placed 

 considerably higher in the series than the conglomerates. The true 

 reading of this section, to our minds, will not admit of an unconfor- 

 mity at the point mentioned by Mr. Maw, but it must be sought 

 for at the junction of the conglomerates with the metamorphic 

 series below, as at Moel Tryfaen and other places in North Wales. 



The curious change in the rocks which he has taken for an un- 

 conformity is probably, as suggested by Prof. Hughes, only an ap- 

 pearance due to rock- structure, not uncommon in beds of unequal 

 texture and extent, cleaved so as to crumple the hard beds and 

 thrust the cleaved slates into the folds. Similar appearances have 



