PEBIDIAN BOCKS OF PEMBROKESHIRE. 159 



At Eamsey Island, associated with a similar lava, there occurs a 

 good breccia in which large masses of hornstone occur imbedded in 

 a felstone matrix, studded with grains of glassy quartz. Here also 

 are seen some bands which appear to be good quartz felsites. 



(10) Succeeding the felstone in the section occurs another series 

 of hard bright-green ash bands, and a rough ash with fragments of a 

 very dark green rock. In this series Mr. Hudleston noticed a good 

 vein of epidote. These beds, as well as some in the other series, 

 are frequently traversed by thick veins of quartz ; and it was asso- 

 ciated with one of these that the epidote was found. Veins of 

 asbestus also have been observed in these rocks. 



(11) The last series exposed in this section is made up of reddish 

 and purplish ashy schists. They are not exposed immediately in the 

 line of the section, but are found on the coast of Eamsey Sound to 

 the south, and at "Whitesand Bay, on the east side, to the north. 

 They are seen at the last place to be covered unconformably by the 

 Cambrian conglomerates, and are here penetrated by a thick trap 

 dyke, which is also overlapped by the conglomerates. 



The line of strike of the Cambrian rocks appears at first sight to 

 be nearly identical with that of the underlying Pebidian beds ; but 

 when examined carefully it will be seen that in no case is it truly 

 so, but that the conglomerates overlap the beds irregularly and at 

 different points in the succession. They usually dip also at a lower 

 angle than the Pebidian beds. Moreover the latter are always more 

 or less highly metamorphosed ; and the Cambrian conglomerates are 

 largely made up of pebbles derived from these rocks in their altered 

 state, imbedded in an unaltered matrix. 



The thickness of the Pebidian rocks as exposed at St. David's 

 may be estimated now at least at 8000 feet. 



In my former paper I only gave it at about 3000 feet, as I was 

 then doubtful whether the series to the west was a continuous one, 

 or whether it simply consisted of repetitions in folds of the beds im- 

 mediately flanking the ridge. Wow, however, it is clear from special 

 characters indicated at various points in the section that it may be 

 looked upon generally as a continuous series, possibly repeated par- 

 tially by slight folds and faults, but yet sufficiently continuous to 

 give at least the thickness I have now estimated it at. Throughout 

 the whole series the bedding is very distinct, and generally at a high 

 angle. As the beds also are overlapped by the Cambrians wherever 

 the latter are not cut off by faults, this estimate may not approach 

 to any thing like the actual thickness of the series if wholly ex- 

 posed ; but it is that which we may fairly claim for it as seen at 

 St. David's. 



In the other areas these rocks show some slight differences in 

 their composition, and may possibly belong to higher positions in the 

 group. The detached mass in Eamsey Island consists chiefly of 

 felstone, felstone porphyry, and hornstone and felstono breccias. 

 The Cambrian conglomerates resting upon this mass are also chiefly 

 made up of rounded felstone pebbles, indicating that they were 

 mainly derived from such underlying rocks ; they are actually the 



