262 A. GEIKIE ON THE SUPPOSED 



proclaimed so loudly and persistently that, in spite of the protests of 

 my predecessor, Sir Andrew C. Ramsay, who will not admit the 

 presence of such rocks in any part of the Principality, I had 

 gradually "been led to believe that they really must exist, though 

 probably not to the extent that had been claimed for them. In 

 visiting Wales, therefore, I went with no prejudice in favour of the 

 views expressed by the Geological Survey. On the contrary, I had 

 a conviction that these views must be, in some measure, at least, 

 erroneous, and that this admission ought to be frankly made. 



I chose the St. David's district as being that about which most had 

 been written, and which had, in a measure, been taken as a typical 

 area for the " Pre-Cambrian " rocks of Wales. It is desirable at the 

 outset that it should be clearly understood that the conclusions to 

 which I have come refer solely to that district, and that, in the 

 meantime, I offer no opinion regarding other so-called Pre-Cambrian 

 areas in the Principality. 



That my examination of the ground might be made in greater 

 detail, I requested my colleague Mr. B. N. Peach, of the Geological 

 Survey of Scotland, to accompany me. His long experience among 

 crystalline rocks of many kinds has given him great practical 

 insight into the structure of these formations in the field. Like 

 myself, he went prepossessed in favour of " Pre-Cambrian " ideas. 



We visited all the sections together, and came to complete 

 agreement in our interpretation of them. The following pages give 

 an account of our joint research in the field, and of my own subse- 

 quent petrographical investigation of the rocks collected by us*. 



The earliest published account of the rocks of St. David's appears 

 to have been that given by Dr. Xidd, in vol. ii. of the First Series of 

 the Society's ' Transactions.' This author speaks of the rocks as being 

 in some instances " stratified," in others " unstratified ;" the hills, or 

 rocky summits, consisting of materials that " bear no marks of 

 regular stratification," but " appear as so many nuclei, about which 

 is arranged a very curiously diversified series of highly-inclined 

 strata of a kind of slate "f . 



The next notice is that of Sir Henry De la Beche, in a paper read 

 to this Society in 1823. He separates the "trap" and "grey- 

 wacke rocks of St. David's, and is inclined to regard the trap 

 as "having been forcibly intruded amongst the other rocks at a 

 period subsequent to their consolidation "J. The areas respectively 



* Since this was written I have made a second visit to St. David's, accom- 

 panied by my colleague Mr. W. Topley, of the Geological Survey of England 

 and Wales, with the object of collecting additional material for the second part 

 of the present paper. I have thus been able fully to confirm the conclusions 

 arrived at on the first occasion, and to obtain some additional evidence in the 

 same direction. But nearly the whole of the data bearing upon the question of 

 the existence of Pre-Cambrian rocks were collected in company with Mr. 

 Peach ; and the first part of the paper had been prepared before my second 

 excursion. 



t Trans. Geol. Soc. ser. 1, vol. ii. p. 79 (1814). 



\ Trans. Geol. Soc. ser. 2, vol. ii. p. 2 (note). 



