Vol. 68.1 PRE-CAMBEIAN AND CAMBRIAN OF PEMBROKESHIRE. 



399 



portion being, in general, andesitic in character, the upper rhjolitic 

 and lieratophyric. 



The Dimetian, which is intruded into the Pebidian, and is 

 therefore younger, includes granite and quartz-porphyry, diorite, 

 and probably a set of basic dykes which are later than the other 

 rock-types. The granite and quartz- porphyry appear to be variants 

 of the same rock, and, together with the diorite, are rich in soda 

 and relatively poor in lime. 



The Cambrian deposits form a much-torn fringe to the pre- 

 Cambrian massif, and for descriptive purposes have been divided 

 into two, a lower division to which we have applied the appellation 

 Welsh Hook Beds and a higher termed Ford Beds. 



Pig. 3. — Index-map of North- West Pembrokeshire , showing the 

 areas occupied hy pre- Cambrian rocks. (Scale : 1 inch=4 miles.) 



[I nil b]ack=Eimetian rocks ; clotted areas = Pebidian rocks. A=Mr. J. F. N, 

 Green's map, Q. J. G. S. vol. Ixiv (1908) pi. xliv ; B — PI. XL accompanying 

 this paper.] 



The "Welsh Hook Beds can be correlated closely with Hicks's 

 Caerfai Series, and, like the similar beds at St. David's, consist of 

 a basal conglomerate, pebbly grit and quartzite, green sandstones, 

 red shales, and purple sandstones. 



The Ford Beds, which are chiefly dark shales, are less easy to 

 place; but they appear to be referable to that portion of the Upper 

 Solva Group which is not far removed from the Menevian. There 

 is, consequently, a stratigraphical break between the Ford Beds 

 and the highest members of the Welsh Hook Group. A large part 

 of the Lower Cambrian sequence, as established by Hicks near 

 St. David's, is absent in this district ; the greater portion of the 

 Solva Series is not represented, being in all probability transgressed 

 by the higher beds between Solva and Hayscastle. 



The basal bed of the Cambrian rests upon a variety of rocks which 

 belong to l)oth the Pebidian and the Dimetian, and it often has 

 a strike which is wholly discordant with the rocks beneath. It 



