Prof. T. McKenny Hughes — The Ffynon Beano Caves. 489 



rock is, however, furnished by its relation to fault-planes. A rock 

 must necessarily be solid before it can be faulted. Now, we find 

 at Pen Voose near Landewednack, that massive gabbro passes over 

 into gabbro-schist at a fault-plane, and that the foliation in the 

 gabbro is such as would be produced by a shearing motion parallel 

 with the fault-plane. Taking all the facts into consideration, we 

 appear to be justified in concluding that the foliation in the Lizard 

 gabbros is the result of pressure- or regional-metamorphism. It 

 seems also probable that the replacement of felspar and diallage by 

 saussurite and hornblende has been largely determined by the same 

 agency. 



DESCRIPTION OF PLATE XIII. 



The Plate represents the appearance of a polished slab of the augen-gabbro of Karak- 

 clews. The " eyes " are formed of diallage. The dark bands are formed mainly of 

 green hornblende ; the white bands, of the aggregate generally known as saussurite. 

 The white spots seen in certain parts of the Plate represent an undetermined mineral 

 which is almost constantly present in the Lizard gabbros. It appears in the glassy 

 felspar of the comparatively unaltered rocks, and increases in amount as the felspar 

 passes into the condition of saussurite. There is much greater detail in the dark 

 bands than is represented in the Plate. The block is traversed by narrow veins which 

 cut the planes of foliation at angles of about 15°. 



II. — On the Ffynon Bettno Caves. 

 By Prof. T. McKenny Hughes, M.A., F.G.S. 



MUCH interest has been recently aroused in the exploration of 

 the caves of the Vale of Clwyd, partly by the enthusiasm of 

 the principal promoter of the investigation, Dr. Hicks, but chiefly 

 owing to the inferences he has drawn from the observations made. 

 When reading the Eeport of the British Association Committee for 

 the Exploration, of which he was Secretary, he assumed the entire 

 responsibility of those theoretical deductions. In these, however, 

 he was supported by all the speakers who followed him on the 

 subject, including the other members of the Committee excepting 

 myself. He had previously anticipated the report by the announce- 

 ment in Nature of his discovery of Pre-Glacial Man in the Yale 

 of Clwyd. I therefore ask leave briefly to state my reasons for 

 differing from his conclusions. 



First, I recognize in the Vale of Clwyd four drifts which must be 

 distinguished in this inquiry. 



1. The Arenig Drift, which contains fragments from the W. and S. 

 only, and which is the only true glacial deposit in the Vale of Clwyd, 

 as there is no evidence of any glacier having ever come down the 

 Vale. 



2. The Clwydian Drift. — (.4) a marine deposit due to the sub- 

 mergence which followed the age of extreme glaciation and contain- 

 ing, in addition to the re-sorted debris of the older drift, flint and 

 fragments of granite and other north-country rocks. (B) the still 

 further winnowed debris belonging to the age of emergence. 



I am not very clear about the possibility of often distinguishing 



