168 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 20, 



Here, however, they are largely interstratified with purple slates. 

 They are in places much contorted, and, on this account, the thick- 

 ness of slaty beds, as wrought in the quarries, appears much larger 

 than in reality it would do, but for the repetition of the same beds by 

 contortion. But little more than 2000 feet of these rocks rise to the 

 surface by the sides of the Lake of Llanberis, whereas more than 

 6000 appear in Merionethshire. 



The slates, I believe, are the equivalents of some of the sandy beds 

 of Merionethshire. The sandy sediment has passed into mud, or 

 vice versd, — a phaenomeuon found to be of constant occurrence when 

 we take in detail the strata of the Caernarvonshire development 

 of these rocks, where slates, sandstones, and conglomerates in many 

 places pass rapidly into each other. The interstratified sandstones 

 and conglomerates are sometimes singularly inconstant. Most of 

 them, even some of the more slaty conglomerates, are highly cleaved, 

 and the pebbles are elongated in the lines of cleavage, — a circumstance 

 remarked by Mr. D. Sharpe in regard to some of the rocks of Cum- 

 berland. Some of the conglomerates of Llanberis, in this the most 

 ancient of the Welsh formations, contain pebbles of quartz, quartz- 

 rock, felspathic trap, and quartz-porphyry, purple slate, black slate, 

 red jasper, &c. They lie among the lowest exposed beds on the 

 banks of Llyn Padarn, and the inference is undeniable that this con- 

 glomerate has been formed from the waste of a set of beds that formed 

 some old land in many respects similar to Wales of the present day. 

 The pebbles of purple slate resemble those of the very strata amid 

 which they are found. The quartz-rock and jaspers resemble some 

 of the metamorphic rocks of Anglesea, which metamorphisra, it will 

 be shown, probably took place subsequent to the deposition of this 

 conglomerate ; and the black slates and felspathic traps and por- 

 phyries are indistinguishable from those that occur about the base 

 of the Bala beds in the district of Cader Idris, the Arans, the Arenigs, 

 and Moel-wyn ; or of the higher Bala beds in the heights of Snowdon, 

 Carnedd Dafyd, and Moel Hebog*. In many of the pebbles of this 

 old conglomerate there is no appearance of metamorphism, in the 

 extreme sense of the term, as applied to foliated rocks ; in others 

 metamorphism may have occurred. 



In a large sense of the term, I know of no formation in England 

 or Wales of older date than these Cambrian slates, sandstones, and 

 conglomerates ; and the facts above described therefore, more than 

 in any other case with which I am acquainted, prove that the same 

 kind of slates, purple and black, of porphyries, and of metamorphic 

 rocks formed some land of which we should have had no knowledge 

 were it not that its former existence is revealed in the structure of 

 these conglomerates. The oldest formation now known in Wales, 

 therefore, does not represent the beginning of those phsenomena of 

 ordinary deposition, of ordinary volcanic action, and of metamor- 

 phism, which last in this, and in other parts of Europe is character- 

 istic of many epochs in time as high at least as the Eocene rocks 



* There can be no mistake about the position of tliese conglomerates. They 

 distinctly underlie the whole of the workable beds of purple Cambrian slate. 



