400 ME. H. H. THOMAS AND PEOF. 0. T. JONES ON THE [Sept. I9I2, 



also contains an assortment of pebbles which in some instances 

 are identical with, and were probably furnished by, the underlying 

 rocks. 



It is, therefore, considered that in the district described the base 

 of the Cambrian Formation rests uuconformably on a complex 

 series of tuffs, lavas, and plutonic rocks. 



Subsequent to the formation of the Pebidian, there were one or 

 more periods of intrusion followed by extensive denudation of the 

 rocks which covered the plutonic masses. There was, consequently, 

 a lapse of a considerable interval of time between the laying down 

 of the Pebidian tuffs and the deposition of the earliest Cambrian 

 rocks. 



The pre-Cambrian and early Cambrian rocks of the Hayscastlc 

 area form a horst surrounded by younger beds. They are faulted 

 on all sides, and the greater number of the faults are probably due 

 to movements which took place in pre-Carboniferous times, and 

 caused a gradual overstep of the Carboniferous rocks on to the older 

 formations. 



Post-Carboniferous movements have also been active in the 

 district, producing faults of some magnitude. 



EXPLANATION OF PLATE XL. 



Geological map of the Brawdy and Hayscastle District (Pembrokeshire), 

 on the scale of 2 inches to the mile, or 1 : 31,680. 



Discussion. 



Mr. J. F. jST. Geeen said that he had listened with the greatest 

 pleasure to this important paper. The country was most difficult, 

 and the complex structure could only be elucidated by the refined 

 methods of mapping adopted by the Authors. The general equiva- 

 lence of the tuffs to those of St. David's was of interest, especially 

 the persistence of the acid Clegyr Conglomerate, which also 

 occurred in the Solva Valley and retained its characters and strati- 

 graphical position for at least 9 miles. 



He thought that more evidence was required before the pre- 

 Carboniferous age of the main movements could be accepted as 

 definitely proved. 



The conclusion that basic dykes of pre-Cambrian age probably 

 occurred was interesting. At St. David's the predilection of the 

 basic dykes for the Dimetian was well-known ; and, near the 

 cathedral, dykes, which seemed to differ from the common post- 

 Cambrian types, were found in both Pebidian and Dimetian : but 

 their pre-Cambrian age, though probable, was not yet demon- 

 strated. 



In conclusion, he wished to express his pleasure in that some of 

 the older rocks of the region had been investigated by modern 

 petrographical methods. 



Mr. G. Baeeow was pleased to see that the main results of 



