396 MB. H. H. THOMAS A^'D PROF. 0. T. JONES ON THE [Sept. I912, 



once more in contact with it : near Brawdy it is fairly certain that 

 if any fault occurs at the junction of the Cambi'ian with the older 

 rocks it is of trifling amount. 



It has been mentioned that pink amethystine quartz -pebbles 

 occur in the conglomerate, and are similar to those in the neigh- 

 bouring quartz-porphyry : also we have commented on the nature 

 and abundance of the igneous boulders in the conglomerate of 

 Silver Hill (p. 391). 



The foregoing facts and considerations render it most probable 

 that in the district described the base of the Cambrian rests 

 unconformably upon a complex series of tuflPs and lavas, and of 

 plutonic masses intruded into these volcanic rocks. 



Subsequent to the period of the formation of the tuffs referred to 

 the Pebidian, there must have been one or more periods of intrusion 

 which were followed by extensive denudation of the rocks that 

 formed the cover of the plutonic masses. These facts point to the 

 lapse of a considerable interval of time between the formation of 

 the Pebidian and the deposition of the earliest Cambrian rocks. 



(c) Post-Cambrian Basic Dykes. 



Basic dykes which cut the Cambrian deposits are exposed at 

 several localities. Two dykes, one much thicker than the other, 

 cut the purple sandstones of the Welsh Hook Beds in the little bay 

 known as Cwm-mawr, at Xewgale, and may be seen on the fore- 

 shore. They are both albitized diabases : the thinner dyke (E 7232), 

 the southernmost of the two, contains good pseudomorphs after 

 olivine (fig. 2A, p. 389). 



The thicker dyke (E 7235) presents chilled margins, now much 

 faulted and sheared, to the Cambrian rocks into which it is in- 

 truded. This rock was noted by Murchison,^ who indicated it in 

 his sections ; he was wrong, however, in showing it as coming up 

 along the fault which separates the Cambrian from the Carboni- 

 ferous ; it lies entirely within the Cambrian formation, and is 

 certainly pre-Carboniferous in age. 



Other dykes which cut the same division of the Welsh Hook Beds 

 may be seen on the eastern side of the Cleddau, north of Welsh- 

 Hook Bridge ; they are much sheared, and completely decom- 

 posed. Originally, they were in all probability diabases similar to 

 those of Xewgale. 



IV. Tectot^ics. 

 The pre-Cambrian and early Cambrian rocks of the Hayscastle 

 area seem to stand in the relation of a horst to the higher beds 

 which adjoin them. The boundaries of the area are faults except 

 in the south-west, where the rocks are truncated by the coast. 

 These faults have in all cases let down younger rocks on the outside 

 of the area, but there is no decisive evidence as to whether they 

 are normal or reversed. 



^ ' Silurian System ' 1839, pi. xxxv, figs. 7 & 9. 



