158 H. DICKS ON THE DIMETIAN AND 



green clay slates, often spotted. These are chiefly seen on the east 

 side of the St.-David's valley going northwards. These and the 

 underlying breccias dip away from the Dimetian axis at a high 

 angle, usually about N.N7W. 



(6) A series of greenish, reddish and purplish indurated ashes and 

 ashy shales. The rougher ashes are frequently conglomeratic, and 

 sometimes contain pebbles of considerable size, the masses being 

 chiefly purple and green shales, felstone, and quartz. The matrix is 

 studded throughout with an abundance of dull and glassy quartz 

 grains and with broken bits of felspar. In the finer ashes, which 

 are sometimes highly indurated, these quartz grains give the mass 

 quite a porphyritic appearance ; and it is impossible not to be im- 

 pressed, when looking at some of these beds, with the opinion that, 

 had metamorphism been carried a little further here, the result would 

 have been, to all appearance at least, a tolerably good quartz 

 porphyry. 



The beds of this series attain a considerable thickness, and may 

 be traced all along the west side of the valley, from a quarter of 

 a mile above the Cathedral to half a mile below near to the Camp, 

 and then across in some road exposures to Clegyr-foia rock. This 

 last rock, which stands out alone considerably above the general 

 surface, consists chiefly of bands of highly indurated fine-grained 

 green ash (Appendix, § 2). which does not weather readily, inter- 

 stratified with softer reddish and purplish shales. 



(7) In the marshy ground beyond Clegyr-foia, the rocks are almost 

 entirely hidden by a considerable thickness of marly clay, the result 

 probably of the gradual decomposition of the underlying rocks, com- 

 bined with an admixture of a certain amount of surface-drift. 

 There are, however, one or two exposures just at entering the 

 moor, which show that the underlying rocks are a continuation, as 

 would be expected by the line of strike, of rocks exhibited in a cliff- 

 section at Porthlisky Harbour to the south of this point. At the 

 latter place they have been brought by a fault to rest directly on the 

 Dimetian axis, but dipping away from it at a very high angle. They 

 consist for the most part of reddish, yellowish, and whitish schists 

 and slates, alternating with beds of volcanic tuff. In some cases 

 they are hard and gneissoid in character, but mostly soft, and decom- 

 posing readily on exposure to atmospheric influences. 



(8) On the west side of the harbour the last-mentioned series 

 is followed at a rather low angle of dip (the effect doubtless of a 

 fault) by conglomerates and ashy beds. The ash-beds and the con- 

 glomerates are much like those described as occurring on the west 

 side of the St.-David's valley, the matrix being also like in con- 

 taining an abundance of quartz grains, sometimes of large size. 



There are also some bands of a reddish ash rather prettily 

 blotched with green and white, the latter being bits of felspar in a 

 state of decomposition. 



(9) A thick band of felstone weathering white, with a brecciated 

 appearance on the exposed surface, but rather compact internally ; 

 this seems to be a true interbedded lava of contemporaneous origin. 



