292 A. GEIKIE ON THE SUPPOSED 



In concluding this part of my paper, I am bound emphatically 

 to declare that the map of the St. David's district, as surveyed by 

 De la Beche, Eamsay, and Aveline, is in its essential features 

 correct. Dr. Hicks has denied its accuracy, and has even gone 

 so far as to assert that " a very cursory examination " suffices to 

 show its errors. One would have thought that something more 

 than a "very cursory examination" would have been required 

 to upset the mapping laboriously worked out by men who, from 

 long years of training, had acquired an almost unrivalled skill in 

 field-geology. At all events we might fairly have expected that, 

 instead of merely declaring the map to be wrong, Dr. Hicks would 

 take every care to show why, after prolonged consideration, he 

 could not accept the conclusions of his predecessors. Their long 

 years of geological experience, I venture to think, entitled their 

 work, whether right or wrong, to more than a summary dismissal. 

 But the same treatment which Dr. Hicks meted out to them in the 

 St. David's area, he has consistently continued in his subsequent 

 excursions over Wales. Having apparently convinced himself — on 

 what grounds I have endeavoured to show — that the rocks coloured 

 on the Survey Maps as felstone or quartz porphyry must belong to 

 his "Arvonian" group (that is to say, are not intrusive in the 

 Cambrian or Lower Silurian strata, but prominences of Pre-Cambrian 

 age), he has proceeded to apply this conviction to the Geological 

 Survey maps all over Wales. With the most complete disregard of 

 the evidence by which the officers of the Survey were led to regard 

 certain rocks as intrusive, he simply turns the felstones, syenites, 

 &c. into metamorphic and volcanic Pre-Cambrian masses*. I have 

 deliberately restricted myself in this paper to the discussion of the 

 St. David's district ; and I therefore offer no opinion as to the validity 

 of the Pre-Cambrian areas cited by Dr. Hicks in other parts of 

 Wales. But I am sure that geologists generally will support me 

 when I contend that it is not by the "cursory examination" of 

 wide areas that the country can be remapped. This was not the 

 style in which the Survey Maps were constructed ; nor is it the style 



* It will be a work of some labour to follow Dr. Hicks in bis rapid tra- 

 verses of Wales, with the view of testing his corrections of the work of 

 his predecessors. Mr. Peach and I had time to visit a few of the areas he 

 has renamed, and always with the same result. Thus, on the coast near 

 Newgale, about eight miles east of St. David's, he describes a mass of Pre- 

 Cambrian beds, chiefly " felstones," "flanked by Cambrian conglomerates con- 

 taining pebbles identical with the rocks below " (Quart. Journ. Geol. Soc. vol. 

 xxxiv. p. 160). .All that we could find was an eruptive rock penetrating 

 and altering black Cambrian shales. Again, he describes the quartz felsites of 

 Eoche Castle as belonging to his "Arvonian" group (Quart. Journ. Greol. Soc. 

 vol. xxxv. p. 286). If bedding exists in this rock, I can see no i*eason why 

 every eruptive rock should not be regarded as bedded, or why, on the same 

 ground, several unconformable Pre-Cambrian formations might not be made 

 out in any good-sized granite-quarry. It is interesting to remember that the 

 true structure and intrusive character of the Trefgarn (Roche-Castle) rook were 

 shown in 1836, by Murchison {ante, p. 263). In the ' Silurian System' (p. 402), 

 he says in reference to it that, " though offering no traces of true bedding, the 

 compact felstone of Trefgarn is divided into rude prisms by two sets of planes 

 or vertical and horizontal joints giving rise to square-topped masses like ruins." 



