﻿374 MR. J. F. N. GREEX ON" THE GEOLOGICAL [Aug. I908, 



constant horizon in the Pebidian (p. 368). It is most important to 

 note that the actual western edge of this prolongation is in contact 

 with, and presumably passes under (for there is no sign of a fault), 

 the member (B 1) of the Pebidian which forms the roof of this silL 

 As the exact position of the remaining boundaries of the Dimetian 

 is not a matter of serious dispute, and the discussion of their 

 nature and meaning with reference to the Cambrian is dealt with 

 later (p. 377), it will be convenient now to give some further details 

 of the sill, in view of its importance as bearing on the relation of 

 the Dimetian to the Pebidian. 



The sill has been traced for a considerable distance, and its out- 

 crop is shown on the map (PL XLIV) ; clear evidence of its intrusive 

 character is seen near Perth Henllys, where it breaks up and interdi- 

 gitates with the beds of its roof (E 1). In thin section the intrusion 

 always shows a parallel structure, and at times a banded structure, 

 the matrix of the strips varying in texture from cryptocrystalline to 

 a fairly coarse mosaic. In this are set porphyritic crystals of quartz, 

 orthoclase, and oligoclase ; the quartz, which is abundant, occasion- 

 ally shows corrosion-channels, but is never rounded like the crystals 

 seen in the quartz-porphyry dykes described below. The ortho- 

 cliise at times shows graphic intergrowth. The quartz-crystals 

 are often broken, the larger being cracked and the different portions 

 sometimes actually separated, but never dragged out. Numerous 

 small jaggedly-triangular pieces of quartz also occur, especially when 

 the rock is finer-grained ; these appear to be the debris of larger 

 crystals. A variable amount of sericitic mica is present in the 

 ground-mass, suggesting a slight amount of cataclastic structure ; 

 but in some specimens the amount is so small, that the foliation of 

 the rock is probably in the main protoclastic and to some extent 

 connected with its viscous nature before consolidation. 



The general petrological resemblance to the marginal modification 

 of the granophyre, and the fact that this modification appears to pass 

 into a rock identical with the sill and at the same horizon, suggest 

 that the latter is simply the tapering edge of a laccolitic intrusion 

 of which the St. David's rock (Dimetian) is the core. If the views 

 here put forward are justified, the granophyre must be intrusive in 

 the Pebidian.^ 



YI. Qtjartz-Poephyey Dykes. 



In the St. David's area, in addition to the granophyre, there are 

 a number of acid dykes (quartz-porphyries), described by Dr. Hicks 

 as in the main of Arvonian age and therefore entirely unconnected 

 with the Dimetian : the view taken by Sir Archibald Geikie was 

 that they are not only connected withj but in some cases apophyses, 

 or outward prolongations of the margins, of the granophyre. The 

 term Arvonian has been dropped by general consent: but, 

 although these dykes are probably a later phase of the granophyric 



1 The rock here described as a sill was included by Dr. Hicks in his Pebidian; 

 a description of it was given by Thomas Davies, Quart. Journ. Geol. Soc. 

 vol. xl (1884) p. 552, nos. 32 & 33. 



