48 Correspondence — Mr. A. B. Wynne. 



by Mr. Maw entirely to rock structure, produced by cleavage on 

 beds of different texture. 



He considered that in the main the Bangor beds were tbe equi- 

 valents of tlie Pebidian of Dr. Hicks, while the Caernarvon beds 

 nearly represented his Dimetian. But he thought there was as yet 

 no proof of an unconformity between these formations. He would 

 explain the apparent unconformity at St. David's by a continuation 

 of bends and faults and joints mistaken for bedding, and would refer 

 the brecciated rock of Low Moor, near St. David's to the Pebidian, 

 thus taking it on the wrong side of the supposed unconformity. He 

 thought that the green beds in the Dimetian were, in all the cases 

 where he had been able to examine them, oi'iginally dykes. 



He saw, therefore, no reason, from an examination of other areas, 

 to suspect any different explanation from that suggested by the ex- 

 amination of the Bangor and Caernarvon district, viz. that we have 

 in the Bangor and Caernarvon beds one great volcanic series, on 

 which the Cambrian conglomerates and grits rest with a probable 

 unconformability. 



An appendix by Prof. Bonney, on the microscopical examination 

 of the rocks referred to, accompanied this paper. 



OOK-iaiESIPOn^IDIElIsrOIE. 



" CONCRETIONARY BANDS " OR " CONGLOMERATES " OF LAMBAT 



ISLAND. 



Sir, — In the paper "On the Borrowdale Series, and Coniston 

 Flags," in the Quarterly Journal of the Geological Society for Aug. 

 1877, p. 479, the authors speak of " Concretionary bands " " called 

 by " the late " Mr. Du Noyer, coarse conglomerates," and according 

 to the late Prof. Jukes, containing " pebbles " with Silurian corals 

 attached. In next page these " conglomerates " or " concretionary 

 bands " are said to form a portion of an ash-breccia series, but no 

 reason is given for what would seem to be an entirely unnecessary 

 correction of the descriptions quoted from Mr. Du Noyer and Prof. 

 Jukes ; nor is it stated why the rocks are referred to as " concre- 

 tionary bands." 



These descriptions, quoted at p. 479, will of themselves show the 

 difficulty of accepting the concretionary nature of the Lambay rock 

 referred to; the matrix being of "black mud," inclosing pebbles of 

 "cleaved slate," "grey grit," "grey limestone," "greenish-grey 

 greenstone," "ash," and "limestone conglomerate inclosing rolled 

 pebbles of greenstone" : some of these fragments supporting attached 

 Silurian corals. 



The unqualified application of the word " concretionary," as an 

 amendment to Mr. Du Noyer's " coarse conglomerate," to such rocks, 

 seems a singular use of the term, though it can scarcely be meant to 

 convey the idea that the writers quoted did not know the difference 

 between concretionary rocks and conglomerates. 



A. B. Wynne. 



MuRUEE, Oct. 1877. 



