GEOLOGY. 289 



the Sierra Nevada. They include, in Wales, great quantities 

 of massive and slaty greenstones, serpentines, chloritic, epi- 

 dotic, and quartzose schists, with argillites and so-called tal- 

 cose slates. The thickness of the portion of Pebidian rocks 

 exposed at St. David's, between the Arvonian and the uncon- 

 formably overlying Cambrian, is, according to Hicks, not less 

 than 8000 feet. 



These pre-Cambrian rocks enter into certain conglomer- 

 ates at the base of the Cambrian in Wales, this being es- 

 pecially the case with the Arvonian orthofelsites. Ramsay, 

 from his former studies about Llyn Padarn, near Snowdon, 

 where the Llanberris slates rest upon these older rocks, with 

 such a conglomerate at the base, in which the matrix is very 

 like the pebbles in composition, was led to regard the under- 

 lying porphyry as but the result of an alteration of a por- 

 tion of the stratified Cambrian series. It is shown, however, 

 by Hicks and by Hughes, that this view is untenable, and 

 that we have nothing more than a conglomerate, of which 

 both pebbles and matrix are derived from an adjacent pre- 

 Cambrian rock. Hunt has verified these observations, and 

 points out the fact that they derive an additional interest 

 from the existence of similar conglomerates, derived from 

 Arvonian orthofelsites, along the coasts of Massachusetts and 

 New Brunswick, which have been in like manner regarded 

 by some as half-developed or inchoate porphyries. These 

 conglomerates in different parts of North America are of 

 Lower Carboniferous, Silurian, and Lower Cambrian ages ; 

 and it is probable, from some recent observations, that there 

 exist near Boston, Mass., still older conglomerates and brec- 

 cias of this kind, which are, perhaps, of Huronian age, and 

 may correspond to those already noticed in the base of the 

 Pebidian or Huronian in South Wales. 



The pre-Cambrian age of the crystalline rocks of Wales, 

 maintained in a general way by John Phillips and by Sedg- 

 wick, is now placed beyond a doubt, and the existence there- 

 in of strata belono-in2C to three distinct series or eras seems 

 established. Hunt has recentlv examined them in each one 

 of the four districts studied by Hicks, and compares the Di- 

 metian to portions of the Laurentian gneisses, while showing 

 the apparent identity of the Arvonian Avith the North Amer- 

 ican halleflintas or orthofelsite- porphyries, and of the suc- 



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