PRE-CAMBEIAN EOCKS OF ST. DAVID'S. 265 



vicinity of St. David's all the rocks of igneous origin are repre- 

 sented as intrusive. 



Until the year 1864 this interpretation of the structure of the 

 district appears to have remained unchallenged. At the meeting of 

 the British Association in that year, however, a suggestion was 

 thrown out by the late J. W. Salter that the syenitic belt of the 

 Survey was a portion of Pre-Cambrian land*. The only grounds 

 given for this suggestion were that the rock is syenitic, and that it 

 does not penetrate the overlying Cambrian strata. Next year 

 Mr. Salter, acknowledging himself to have been mistaken, stated 

 that Dr. Hicks had found portions of schist entangled in the syenite, 

 as well as altered strata on the south side of the latter rockf. 



In the year 1871 appeared a joint paper by the late Professor 

 Harkness and Dr. Hicks on the " Ancient Bocks of St. David's 

 Promontory "%. A foot-note in this paper contains an announcement 

 by Dr. Hicks that he had subsequently found on the ridge of 

 St. David's evidence of bedding in its component rocks, and that, as 

 the strike is discordant to that of the Cambrian strata, there must 

 be here a more ancient group of rocks than the Cambrian, occupying 

 a position equivalent to that of the Laurentian group of Canada. 



In 1875 Dr. Hicks asserts more confidently the Pre-Cambrian 

 age of the rocks of this ridge, denies that these rocks are syenite as 

 coloured by the Geological Survey, but maintains that they are bedded 

 rocks — quartz-conglomerates and dark-green shales, partly meta- 

 morphosed, — and affirms that they are covered unconformably by 

 the Cambrian series §. 



Returning to the subject two years later, Dr. Hicks showed a still 

 wider divergence from his original opinion. He now states that he 

 can recognize two distinct series of Pre-Cambrian rocks at St. 

 David's, giving the name "Dimetian" to what he supposes to be 

 the older, and " Pebidian " to the younger series. Discarding the 

 identification of any part of these rocks with syenite, he describes 

 the " Dimetian " as composed chiefly of compact quartz-schists, 

 chloritic schists, and indurated shales, and the " Pebidian " as con- 

 sisting mainly of indurated shales, often porcellanitic in character. 

 He regards the " Pebidian " as resting unconformably upon and 

 partly derived from the waste of the " Dimetian " rocks. The 

 Cambrian beds are stated to lie unconformably on both these series 

 and to contain abundant fragments of themll. A little later Dr. 



* Rep. Brit. Assoc, for 1864, Sections, p. 64 ; Geol. Mag. vol. i. p. 289. 

 The Rev. W. S. Symonds, however, claims to have first suggested to Salter a 

 Pre-Cambrian origin for the St. David's rock. See his ' Records of the Rocks,' 

 p. 31, 1S72. 



t Geol. Mag. vol. ii. p. 430. 



\ Quart. Journ. Geol. Soc. vol. xxvii.p. 384. For the introduction of bedding 

 into the crystalline rocks of the ridge, as expressed in this paper, Professor 

 Harkness does not appear to have been directly responsible. See footnote on 

 p. 387 above referred to. 



§ Quart. Journ. Geol. Soc. vol. xxxi. p. 167. 



| Quart. Journ. Geol. Soc. vol. xxxiii. p. 229 (1877). 



