Yol. 62.] IGNEOUS AND SEDIMENTARY ROCKS OF LLANGYNOGL 233 



(a) The Eastern Side of the Dingle. 



(i) The Lower Andesitic Series. — In the main road, at a 

 point about 300 yards north-west of Coomb House, these rocks may 

 be seen to emerge from below the Old Red Sandstone and to dip 

 northward at 52°, and, on the side of the dingle, north 20° west 

 at 45°, and may be followed upward and northward for about 

 200 yards. They consist largely of banded tuffs, with occasional 

 vesicular and hne-grained-andesitic lavas, which are well-exposed 

 on the steep slope between the main road and the stream to the 

 west. Good vesicular rocks and more compact lavas may be seen 

 to the west of the bend which the road takes before it reaches the 

 Old Red rocks. This volcanic series nowhere exhibits its lower 

 limit, but the calculated thickness of the rocks seen is at least 

 350 feet. 



(ii) The Ehyolites. — Succeeding the above is a series of 

 rhyolites from 140 to 150 feet thick, consisting of pale-yellow, 

 grey, or white-weathering rocks, often minutely but markedly 

 spherulitic even in the hand-specimen. These rocks follow the 

 dip and strike of the andesites below and succeed them with 

 absolute conformity. The highest bed consists of a beautiful 

 rhyolitic breccia, made up of fragments which range up to an inch 

 or more in length, and are similar in all respects to the rhyolites 

 below. This breccia forms a well-marked feature, and a series 

 of crags, running from the main road, down the slope, to the 

 stream ; it is easily located, as it is almost due east of Tre-hyrn 

 farmhouse. 



(iii) The Upper Andesitic Series. — Immediately above the 

 breccia which forms the summit of the rhyolites comes a series of 

 andesitic tuffs and lavas, with an estimated thickness of about 

 900 feet, and identical in appearance with those of the lower series. 

 These beds are best seen in the wood running alongside and above 

 the road, and are exposed from the rhyolites northward to beyond 

 Llwyn-celyn. The first exposure occurs immediately above the 

 breccia, and is seen to consist of a fine-grained, compact, green tuff. 

 The road-bank and the wood to the east, between the breccia and 

 the tributary stream which flows into the dingle at Llwyn-celyn, 

 are occupied by banded tuffs and lavas, chiefly vesicular. The 

 beds dip northward at from 40° to 50°, and the section seen along 

 the road and in the wood, from the limiting-fault on the north 

 to the rhyolitic breccia on the south, is as follows, in descending 

 order : — 



Thickness in feet. 



Banded tuffs 90 



Compact tuffs and andesite 20 v 



Banded tuffs 65 



Tuffs and obscure ground 195 



Banded tuffs 105 



[Tributary stream at Llwyn-celyn.] 



Banded tuffs 70 



Vesicular andesite 7 



