﻿380 MK. J. r. N. GKEEN ON THE GEOLOGICAL [Aug. I908, 



substantially contemporaneous with the P(3bidian, and in view of 

 what has gone before would therefore be pre-Cambrian. Evidence, 

 however, has been obtained tending to prove that the latter are also 

 post-Cambrian, and further, that they simply represent different 

 modes of occurrence of the same material. 



The rocks here described are striking features of the scenery west 

 of St. David's, where they project like sea-stacks above the general 

 rough platform-level of this part of the district. The material 

 composing the stacks is of much finer texture than that of the more 

 conspicuous masses near the north coast recently described by 

 Mr. J. V. Elsden ; in addition, they are locally vesicular, and these 

 features have presumably led to their being described as lavas. 



The small patch forming the stack nearest St. David's (Clegyr-foia) 

 seems to be intrusive, but its age must clearly go with that of the 

 more important mass on the west. The extent and mode of occur- 

 rence of the latter are shown on the map (PL XLIV) ; it is nearly 

 2 miles long, and, as its trend is roughly parallel to the strike of 

 the Pebidian, it gives at first sight the impression of a sill or lava. 

 Its true nature can be determined by examining its margins. Its 

 junction with the Pebidian is very irregular, and is best exposed on 

 and about Rhoson Crags. Here the continuity of the mass is broken 

 by a long narrow strip, partly enveloped, of Pebidian, seen on the 

 northern and eastern flanks of the crag, the junction being approxi- 

 mately parallel to the stratification of the tuff's ; but on the south 

 the margin makes a right-angled bend, and is seen on the bare 

 rock -face to cut across the bedding for a distance of some 15 yards. 

 It is thus clearly intrusive in the Pebidian. 



As the mass is traced from north to south it is soon seen that, 

 where the path of a fault known to cut the bedded rocks is crossed, 

 no trace of the fault occurs in the basic igneous rock, raising at 

 once a suspicion that it is even later than the post- Cambrian 

 faulting. All doubt of the post-Cambrian age is dispelled by an 

 examination of the coast-section at Carn-arwig. The cliff's here, for 

 about a quarter of a mile, are exclusively composed of the same 

 basic material, except at one locality ; here the basal Cambrian 

 conglomerate also occurs, as several (at least six) more or less 

 lenticular masses completely enveloped in the igneous rock, which 

 must thus be post-Cambrian. Three of these masses are shown in 

 the accompanying photograph (fig. 2, p. 381), kindly taken, after 

 considerable trouble, by Mr. John Barrow. 



If the coast-section near the main mass of basic rock is examined, 

 the fault-planes and ramifying cracks are often seen to be filled 

 with basic material substantially identical in structure and com- 

 position with portions at least of the mass inland. These would 

 seem to be films given oif from the margin of the mass, and this 

 view is strengthened by the fact that, although these small 

 intrusions are very numerous in the district, they have not been met 

 with piercing the larger masses. 



Thus the field-relations do not support the view that these basic 

 rocks are lavas, as has formerly been supposed ; neither does the 



