﻿372 ME. J. F. >r. GEEEN OX THE GEOLOGICAL [-^^g. I908, 



Owing to the readiness with .which it decomposes, good natural 

 exposures are not common, but an excellent section may be seen 

 in the banks of the Alan Kiver, three-quarters of a mile north 

 of St. David's. The total thickness, where measurable, is about 

 200 feet. 



The next division (D 2) consists mainly of schistose rocks, built 

 up of red and green lenticles containing small 'eyes' of variable 

 composition. These eyes are commonly composed of felspar or 

 quartz, but occasionally of felsite, altered trachyte, or magnetite. 

 The lenticles in which they are set consist essentially of chlorite and 

 sericite in minute scales. Interbanded with the above is always a 

 fissile pale-green or yellow rock, flecked with olive-green. A good 

 exposure occurs near Carn-arwig, in the cliffs of Eamsey Sound, 

 where the thickness is about 300 feet : a little farther north the 

 gradual passage to the underlying beds is clearly exposed. The 

 section of these beds at Porthlisky has been described and referred 

 to by various authors, notably Sir Archibald Geikie, who recognized 

 their identity with the rocks of Eamsey Sound ; but the amount of 

 shearing here is quite exceptional, and consequently they do not 

 afford good material for the study of the original characters of the 

 rock. 



This division is overlain by purple or yellow^ porcellanites (D 3), 

 sometimes containing quartz-grains. They seem to be about 

 120 feet thick at Porthlisky, but are so torn out by shearing that 

 xio trustworthy measurement can be made. Slides of these rocks 

 from Carn-arwig and the city of St. David's resemble those of 

 the Caerbwdy Series ; the broken felspars, however, undoubtedly 

 include andesine, not recognized in the latter. 



The highest tuffs shown in the district (D 4) form the north- 

 eastern cliffs of Porthlisky. They consist of soft, pulverulent, 

 schistose rocks of various pale colours, with a tendency to silicifica- 

 tion in strings and patches. They differ markedly from any other 

 part of the Pebidian, and are seen only at this locality. 



In view of the great age of the Pebidian, the preservation of the 

 original characters of many of the fragments in the tuffs is remark- 

 able ; thus the orthoclase-trachyte fragments in the Treginnis 

 Series are almost as fresh as some of the Tertiary trachytes in Skye, 

 described by Mr. Harker. The red rhy elite from the same series 

 shows good flow-structures and unaltered felspar - phenocrysts ; 

 while occasional fragments of augite-andesite are less altered than 

 the post-Cambrian intrusions described later, the felspar-laths being 

 easily determinable. The chief alteration of the tuft's consists in 

 the silicification of the matrix of the acid members, which is so 

 marked that it seems to have been taken as evidence of thermometa- 

 raorphism due to quartz-porphyries. 



