part 1] IGNEOUS AND ASSOCIATED ROCKS OF LLANWRTYD. 27" 



3. Spilites 



I 



y 100' 



Approximate thick- 

 ness in feet.. 

 fSpilites showing' typical pillow-structure,"^ 

 with associated cherts and ashes. The 

 latter are frequently quite coarse, but are 

 intimately associated with the lava. One 

 can distinguish two flows in the upper part, 

 separated by a few feet of ash. 

 Spilite-breccia (see above, p. 20). The beds 

 exposed in the gully on the north at this 

 horizon seem to be mostly ashes, but the | 

 spilite-flows are seen both on the east and 

 on the west — only about 150 yards in each | 

 case. J 



TNot exposed on the southern side. On the ^j 

 ~ q, 1 J north they consist of hard, black, uncleaved I 



| mudstones with fossils in the upper part f 



[_ (Dicranograptus). J 



Coarse ashes and breccia, passing- down into V 

 a coarse breccia of rhyolitic appearance. I 

 These form almost vertical cliffs on the j 

 northern side of the valley. J 



Fairly coarse ashes, with white fragments of 

 [_ felspar in a greenish matrix seen for 



1. Lower Ashes 

 and Breccia. 



m 



110' 



15. 



Total. 



515- 



(5) The Valley of the River Irfon (northern side). 



The section across the anticline shown in fig. 2 (p. 30) is taken 

 a short distance north of the valley of the Irfon, and represents 

 fairly closely a view of the rocks exposed on the northern flank of 

 the valley itself. The spilites are the lowest beds seen, and form 

 a somewhat rugged cliff overlooking the river. The lower part 

 consists of very large ' pillows ' — some measure more than 15 feet 

 in diameter ; while the pillows in the upper part are smaller, and 

 more normal in size. The sediments which succeed the spilites 

 are not clearly exposed, but seem to consist almost entirely of 

 hard banded grits, tough blocky mudstones, and indurated shales. 

 Some 40 feet below the summit a band of ashy limestone (or, when 

 weathered, 'rottenstone ') is found. It crops out in a tiny crag on 

 the west side of the hill, about 20 yards from the conspicuous 

 crags made by the Upper Ashes. The latter are well exposed, and 

 here clip at a high angle westwards. The succeeding banded ashes 

 are better seen on the eastern side of the hill. They are exposed 

 in the quarry marked on the map (PI. II), where one finds the 

 hardened graptolitic shale yielding Dicelloc/raptus, etc., in the, 

 upper part. 



(c) The Valley of the Nant Cwm-du. 



The rocks specified below are exposed in the bed of the stream,. 

 and are also seen pitching northwards on the hill-slope to the 

 south : — 



TT . , f Fairly coarse 



Upper Ashes i Fine 'hornstc 



ashes, 

 hornstone.' 



