284 A. GEIKIE ON THE SUPPOSED 



overlapping of the sections by the Cambrian rocks, much is hidden 

 from view " *. 



I accept generally the lithological descriptions here given, but 

 with important modifications to be afterwards stated. There can 

 be no doubt that the group is almost entirely of volcanic origin — 

 formed principally of various tuffs with bands of olivine diabase and 

 occasional intrusive masses of quartz porphyry. I have already 

 shown that the volcanic nature of these rocks was clearly recog- 

 nized by my predecessor, Sir A. C. Kamsay, in his original map and 

 section, and in his early MS. report on the St. David's area, and that 

 he afterwards allowed this view to be set aside in favour of the 

 opinion that the peculiar bedded rocks on the west side of the granite 

 ridge are altered Cambrian strata through which intrusive " green- 

 stones " have been injected. It is this view which is expressed upon 

 the second and latest edition of the Survey Map and Section. I -at 

 once acknowledge that in this respect the present Map and Section 

 are seriously in error, and that Dr. Hicks deserves the thanks of 

 geologists for having, as it were, rediscovered probably the oldest 

 group of palaeozoic volcanic masses yet known in this country. 



Beserving for the second part of this paper what I have to add 

 to the published descriptions of these rocks, I proceed at once to 

 consider the evidence for their forming a distinct Pre-Cambrian group 

 lying unconformably on the groups below, and covered unconforma- 

 bly by the Cambrian strata above, as has been so repeatedly asserted 

 by Dr. Hicks. 



In this instance, again, I have been unable to discover in his pub- 

 lished papers references to any sections where the proof of the 

 alleged unconformability between the so-called " Pebidian " and 

 " Arvonian n rocks can be seen. The unconformability, if it existed, 

 might be proved (1) by actual sections showing the line of 

 junction, (2) by detailed mapping of the ground and the detection 

 of proofs of overlap and discordance, or (3) by the evidence of in- 

 cluded fragments. Dr. Hicks asserts that " resting unconformably 

 upon the whole [Arvonian Group] are the great agglomerates of 

 Clegyr Hill, the base-beds of the Pebidian, which are made up of 

 masses of Dimetian rocks, of quartz felsites, spherulitic felstones, 

 and halleflintas, and all in the condition in which they are now 

 found composing the underlying ridges. From this evidence it is 

 tolerably clear that the position of the Arvonian or halleninta group 

 is intermediate between the Dimetian and the Pebidian, and that 

 there is, at least in this area, very clear proof of unconformity and 

 hence of lapse of time having intervened " f . In this passage he 

 enumerates two of the three kinds of proof just referred to as 

 indicative of the discordance in question. 



Again my companion and I sought diligently for any trace of the 

 alleged evidence, but completely without success. Like the other 

 assertions with which I have been dealing, its groundlessness became 

 more apparent at every step of the investigation. There is not only 



* Proc. Geol. Assoc, vol. vii. p. 63. 



t Quart. Journ. Geol. Soc. vol. xxxv. pp. 289, 290 (1879). 



