OK THE D1METIAK AND PEBIDIA* E0CES OE PEMBROKESHIRE. 153 



14. Additional Notes on the Dimetia* and Pebidia* Hoces 0/ 

 Pembeokeshire. By Hekry Hices, Esq, P.G.S. With an 

 Appendix by T. Dayies, Esq, F.G.S. (Read December 5, 



1877.) 



mn?! I ™ 61 , W 0n the Pre -Cambrian rocks of St. David's 



£rve a S nm H f ! l° g r l S0C ' V ° L miii ' P' 229 ^ l ™ a ble to 

 give a tolerably full description of the rocks which formed the 



fS ?I r gG ' ° r ^ 1S \ ° f the Promontory; but of those in some of 



the other areas the descriptions, from want of sufficient time for 



.rlfiT ' were necessarily imperfect. Another three weeks 



s- pent last summer m further researches in these areas, enabled me 



t* note many additional facte in regard to tbeir distribution and 



iCZ^U T,l W ^ Gh * h ° pe may be ° f some interest ^d use 



ct^Jl °/i G ^° Cie - y *. These results also S rea % M to the 



fcft A** de ^ tions already given, and render an at- 



tempt at a correlation with other districts, I think, possible. 



J DlMETTAN. 



The additions made to our knowledge of these rocks go to show 

 "their extension into the area between Brawdy and Hayscastle, 

 ten or twelve miles to the east of St. David's. They form an axis 

 to the newer rocks m that neighbourhood similar in many respects 

 to the one at St. David's; and they are there flanked in part by 

 metamorphic Pebidian, and along both sides by unaltered Cambrian 

 rocks ihis portion of the country is so much covered over by drift 

 that it is difficult to meet with many good sections. But whenever 

 these rocks (coloured as granite in the Survey Maps) are exposed 

 they are generally like in character to those forming the central 

 axis at fet David's, especially to the more highly altered or granitoid 

 parts oi the series. 



About Brawdy and Asheston tbe rock weathers readily into a 

 rough granular material with a yellowish staining, similar to that 

 which occurs in beds of the other axis in a quarry at Porthclais and 

 under the Camp, both in the valley south of St. David's 



The following appear to be the chief rocks which make up the 

 a et 'f\ serles ' as exposed in one or other of these neighbourhoods • 

 and added are some of the chief localities where they may be ex- 

 amined Pop ready reference I will describe them as they may be 

 traced from the fault about a mile north of St. David's to the coast 

 about three miles to the south-west. 



1. Quartz porphyries, containing frequently perfect quartz crystals 



(double pyramids), subangular masses of quartz, and crystals of felspar 



m a matrix of greyish or greenish felspathic material (vide Appen- 



l Vi- y Seem t0 extend right across the ridge from west to 



east at this point, and they therefore form a good base for our sec- 



n t nl % Q P°J\ sibl y intrusive, though appearing distinctly to 



H. J.ur. b. JNo. 134. M 



