270 



A. GEIKIE ON THE SUPPOSED 



In reading Dr. Hicks's papers I have been unable to find descrip- 

 tions of, or references to, the numerous natural sections where the 

 relations of the crystalline rock of the ridge to the surrounding 

 mineral masses are displayed. He states, indeed, that the rock is un- 

 conformably overlain by all later formations, but he does not men- 

 tion any localities where he has observed this unconformability. In 

 one passage he speaks of the Cambrian conglomerates resting imme- 

 diately on the " Dimetian " rocks, at the bend in Porth-clais valley, 

 and of higher Cambrian beds in a similar position in the harbour*. 

 It would be more correct to say that the " Dimetian " rock rests 

 there on the conglomerate, as I shall afterwards point out. In 

 another paper he states that " the junction of the Arvoirian with 

 the Dimetian may be seen at St. David's, about a quarter of a mile 

 to the south of the Cathedral, and near Rock House" f. But he 

 immediately adds that a slight depression probably marks a fault 

 at that locality. Mr. Peach and I found on examination no evi- 

 dence of any fault, nor of any line of demarcation between two 

 formations. I shall have occasion to refer to this locality in a later 

 part of the present paper. 



Dr. Hicks, in his various memoirs, introduces many lines of fault, 

 of which, after diligent search, we could discover no trace on the 

 ground, and which, for a clear understanding of the structure of the 

 district, are not required*. 



An unconformability is so important a fact in the geological 

 history of a region, that the most convincing proof of it ought to be 

 demanded. We are entitled to expect that, unless where it is too 

 clear to be mistaken, every available fragment of evidence regarding 

 it should be produced. Still more must this expectation be fulfilled 

 where the rocks in question have been greatly disturbed. No one 

 who has not practically tried it can realize the difficulty of the 

 problem to determine whether or not an unconformability exists be- 

 tween two groups of rocks both of which have been intensely pli- 

 cated or fractured. But to this difficulty no allusion is to be found 

 in Dr. Hicks's papers. 



Mr Peach and I began our work by an examination of the ridge 

 of which Bryn-y-Garn is the crest. We were unable to detect any- 

 where a trace of a structure which had the remotest resemblance to 

 the foliation of gneiss or schist. Nor could Ave discover in the mass 

 any alternations of other rocks. On the contrary, it everywhere 

 retained the same general aspect, and presented all the familiar 

 external characters of a massive eruptive rock. The presence of 

 gneissic structure and intercalations of schistose bands, however, had 



* Quart. Joum. Geol. Soc. vol. xxxiii, p. 231 (1877). 



t Quart. Journ. Geol. Soc. vol. xxxv. p. 289 (1879). 



\ In his map published in 1877 (Quart. Journ. Geol. Soc. vol. xxxiii. 

 pi. x.) Dr. Hicks represents a fault at every locality where the junction of the 

 " Dimetian " with the other rocks is actually visible. These supposed faults 

 have been introduced owing to a mistaken notion of the structure of the ground, 

 and are not required even on the theory to support which they have been in- 

 voked. 



