Correspondence — Lieut.- General C. A. McMahon. 383 



is overlain by the Llandeilo Shales. The author concludes that the 

 andesites, andesitic ash, rhyolites, and diabase-porphyrite are of 

 Lower Llandeilo age ; and that the diabases are post-Llandeilo and 

 pre-Llandovery. 



8. " On the Relations of some of the Older Fragmental Eocks in 

 North-west Caernarvonshire." By Prof. T. G. Bonney, D.Sc, LL.D., 

 F.R.S., F.G.S., and Miss Catherine A. Eaisin, B.Sc. 



In a recent paper on "The Felsites and Conglomerates between 

 Bethesda and Llanllyfni, North Wales," it is argued that, in the 

 well-known sections on either side of Llyn Padarn, a great uncon- 

 formit}'^ separates the rocks into two totally distinct groups. 



The authors of the present communication discuss at the outset 

 the great physical difficulties involved in this hypothesis. 



They further affirm, in the course of a description of the sections, 

 which are most clear and afford the best evidence : — 



(1) That the strike in both the supposed rock-groups is similar. 



(2) That the same is true of the dips. 



(3) That very marked identity of lithological characters may be 

 found in rocks on either side of the alleged unconformity, specimens 

 occasionally being practically indistinguishable. 



(4) That in no case which has been examined can any valid 

 evidence be found in favour of the alleged unconformity, and that 

 in the one which is supposed to be the most satisfactory proof of it 

 the facts are wholly opposed to this notion. 



ON THE ALLEGED CONVERSION OF CHLORITE INTO BIOTITE. 



Sir, — The discussion between Dr. Callaway and myself, on the 

 above subject, has reached a stage beyond which I do not think 

 it can be proiitably carried. 



My great difficulty in this controversy has been to know where to 

 have my adversary. Hunting Dr. Callaway's theories, from paper 

 to paper over the leaves of the Geological Magazine, takes my 

 memory back to certain sporting episodes in my life connected with 

 the Indian Gazelle. These graceful but restless little creatures are 

 constantly on the move, and object, in a provoking way, to stand 

 still to be shot at. I hope I may be pardoned for saying that Dr. 

 Callaway's theories display a similar unsteadiness under fire. 



I understood the author, in his first paper,^ to be stating a case of 

 dynamo-metamorphism, and treated the matter from that point of 

 view. Dr. Callaway replied plaintively that "for the past four or 

 five years, I have been insisting that, in the Malvern crystallines, 

 biotite has been produced out of chlorite by ' contact-action ' ; " - 

 and on the following page he added, " I think it probable that there 

 is not a scrap of biotite in the crystallines of the Malverns which 

 has been produced except by 'contact-action.'" 



On reading these statements, which seemed precise and definite, I 

 tried another shot. Alas ! the bullet flew wide of the mark. The 

 1 Geol. Mag. December, 1893, p. 535. « j^j^. May, 1894, p. 217. 



