266 A. GKEIKIE 01* THE SUPPOSED 



Hicks recognizes that his " Pebidian " series is in great part made 

 up of volcanic tuffs and agglomerates*. 



In 1878 the late Mr. E. B. Tawney, who, in company with 

 Professor Hughes and Mr. Hudleston, went over the St. David's 

 district under Dr. Hicks's personal guidance, gave an interesting and 

 valuable description of the rocks, to which I shall have occasion to 

 make repeated reference in the sequel. In this paper he accepts 

 generally Dr. Hicks's conclusions, but, though classing the crystalline 

 rock of the axis as metamorphic, confesses that " over a portion of 

 the area, at any rate, it did not show evident bedding enough to 

 prevent our classing it as a massive crystalline rock"f. 



In the same year, Dr. Hicks announces the discovery of what he 

 terms a new group of Pre-Cambrian rocks, named by him " Arvo- 

 nian"J, consisting of "breccias, halleflintas, and quartz-felsites," 

 previously included by him in his " Dimetian and Pebidian," but 

 now regarded as intermediate between them, and unconformable 

 with both. In later papers he summarizes the results of his re- 

 searches §. 



During the progress of Dr. Hicks's researches, the rocks of St. 

 David's have been referred to by other writers, sometimes on his 

 authority, sometimes from personal observation under his guidance. 

 It is not necessary to augment this bibliographical outline by citing 

 all such references. Some of them are quoted in subsequent pages, 

 where also several of Dr. Hicks's own papers are dealt with more in 

 detail. 



The object of the present communication is twofold, — first to 

 discuss the evidence for the assertion that Pre-Cambrian rocks exist 

 at St. David's, and secondly to lay before the Society an outline of 

 what appears to me to be the true structure and geological history 

 of that district. 



Paet I. 



The first part of the paper is unavoidably controversial. I pro- 

 pose to examine the evidence for the alleged presence of Pre-Cam- 

 brian rocks at St. David's, and to state the facts which, when brought 

 to the notice of geologists, will, I think, be admitted completely to 

 disprove the existence of any such rocks at that locality. Disliking 

 controvers}^ so much as I do, it is with extreme reluctance that I 

 now enter upon it. But I am sure that, in the interest of truth, Dr. 

 Hicks himself, whose published views I must oppose, will be glad 

 that these views should be subjected to the most searching criticism. 

 It is due to him, no less than to my colleagues on the Geological 

 Survey, whose opinions he has controverted, that I should enter 



* Op. cit. vol. xxxiv. p. 153 (1878). [Their volcanic origin appears to have 

 been first pointed out to him by Mr. Hudleston.] 



f Proc. Bristol Nat. Soc. new ser. vol. ii. part ii. p. 121. 



| Eep. Brit. Assoc, for 1878, p. 536 ; Quart. Journ. Geol. Soc. vol. xxxv. 

 p. 285 (1879). 



§ Popular Science Eeview, N. S. vol. v. p. 289 (1881) ; Proc. Geol. Assoc, 

 vol. vii. pt. i. p. 59 (1881). 



