On the Dimetian and Pebidian Rocks of Pembrokeshire. 157 



2. "Additional notes on the Dimetian and Pebidian Rocks of 

 Pembrokeshire." By Henry Hicks, Esq., F.G.S. 



The additional facts communicated by the author show that at a 

 distance of about 10 miles to the east of the Dimetian axis of St. 

 David's there is another ridge of these rocks, which also runs nearly 

 parallel with it. This is also flanked by Pebidian and Cambrian 

 rocks, and made up of rocks like those in the St.-David's axis. 



The Dimetian formation, so far as it is at present known, consists 

 chiefly of the following rocks : — 



1. Quartz porphyries, containing frequently perfect quartz crys- 

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

 felspar in a felspathic matrix. 



2. Pine-grained greyish quartz-rocks, very compact, and inter- 

 stratified with the above. 



3. Ashy-looking shales of a dull green colour, sometimes highly 

 indurated, but usually showing lines of lamination. Microscopi- 

 cally these show basaltic characters, and are probably greatly altered 

 interbedded basaltic lavas. 



4. Compact granitic-looking rocks. 



5. Quartziferous breccias. 



6. A series of compact quartzites and crystalline schists, inter- 

 stratified with green and purple altered basaltic lavas with a slaty 

 and schistose foliation, and with some dolomitic bands. 



Of the Pebidian formation new areas were added, and the por- 

 tions described in the author's previous paper were further extended, 

 and details as to the chief mineralogical characters given. At the 

 base of the series resting unconformably on the Dimetian is seen an 

 agglomerate composed of large angular masses of a spherulitic fel- 

 stone, pieces of quartz and quartzites, indurated shales, crystalline 

 schists, &c, cemented together by a sea-green matrix of felstone. 

 These are followed by conglomerates of the same materials, which 

 are again succeeded by indurated shales, often highly porcellanitic 

 in character, with a conchoidal fracture. 



These are followed by a thick series of silvery- white and purplish 

 shales and green slates, alternating with fine and rough ashes, often 

 conglomeratic, hornstone breccias, felstone lavas, &c. 



The series, as exhibited at St. David's, has a thickness of over 8000 

 feet; and as it is everywhere, so far as yet seen, overlapped uncon- 

 formably by the Cambrians, it may probably be of much greater 

 thickness. It evidently consists very largely of volcanic materials, 

 at first derived from subaerial, but afterwards from submarine vol- 

 canoes. These materials, however, were also undoubtedly consider- 

 ably aided by sediments of a detrital origin. 



The whole series shows that the sediments have undergone con- 

 siderable changes, but yet not sufficient to obliterate the original 

 characters, and the lines of lamination and bedding are usually 

 very distinct. That they were altered nearly into their present 

 state before the Cambrian sediments were deposited upon them 

 is clear from the fact that the pebbles of the Cambrian conglo- 

 merates which rest immediately on any portion of the series are 

 almost invariably made up of masses of the rocks below, cemented 

 by gritty materials on an unaltered matrix, and from which the 



