Yol. 50.] OF THE NEIGHBOURHOOD OF BTJILTH. 573 



Another area of andesitic ash stretches from just north of Caer 

 Einon and Caer Fawr to near Penrubulla. In hand-specimens this 

 is sometimes dark in colour, hut often light grey or bluish, and 

 occasionally shows distinct white felspars : in some cases, as for 

 instance near the northern diabase-porphyrite, it is mixed with a 

 certain amount of sedimentary material, and is well stratified. 



Sections taken from the three following spots are almost iden- 

 tical : — (1) 140 yards north-east of Penrubulla; (2) the 'Camp' 

 west of Cwm-berwyn ; (3) ' Old Quarry ' east of Newmead Farm ; 

 these show numerous fragments of plagioclase-felspar of various 

 sizes and usually irregular in outline ; the groundmass contains a 

 large quantity of a yellowish-green chloritic mineral and a fair 

 amount of secondary quartz, the latter sometimes replacing the 

 felspars. In some parts of the slides there are groups of very 

 small (usually oval) vesicles with dark borders, and containing the 

 yellowish-green chloritic mineral, or, occasionally, quartz. A few 

 small fragments of andesite are also present. Another slide from 

 just north-west of Caer Einon is similar, but the rock is more 

 decomposed and contains a large quantity of calcite. A section 

 from a little more than | mile north of the summit of Caer Einon, 1 

 and another from i mile north of P>wlch-y-trawspen and \ mile 

 south-south-west of the ' Camp,' differ from those just described 

 in containing very little chlorite, and in the felspars being larger 

 and more numerous. 



There is a small area of ash just south of Big Wood ; in it3 

 southern part this is dark in colour and rather coarse, but north- 

 wards it becomes lighter and finer-grained. A specimen collected 

 at 70 yards south of Big Wood is very similar to those described 

 above from Penrubulla, etc., except that it contains very few vesicles. 

 A section from the southern part of Big Wood (almost clue west of 

 Maen-cowyn) differs in having the felspars much smaller : in another 

 from | mile south of Big Wood, small elongated vesicles are very 

 numerous in some parts of the slide. 



V. The Bhtolites. 



In this district the rhyolitic rocks cover only a small area ; they 

 occur as isolated patches between Maen-cowyn and Penrubulla. 

 In hand-specimens they are usually compact and of a dark grey 

 colour, or, when weathered, yellow or white, and often contain 

 porphyritic felspars. Some of these rhyolites may possibly be 

 intrusive. 



A section from a specimen collected i mile south-south-west of 



1 At a spot 5 mile north-north-west of the summit, of Caer Einon, I found 

 a large block of black chert ; it was not in situ, but I think it could not 

 have travelled more than a very short distance. Under the microscope this 

 chert is remarkable for the large size of the sponge-spicules which it contains, 

 some of them being 5 mm. in diameter. Dr. Hinde kindly examined tbe slides, 

 and he informs me that the spicules belong to the ancboring-ropes of hexacti- 

 nellid sponges, and that there is a piece of Llandeilo Shale from near Builth 

 in the Museum of Practical Geology containing similar spicules. 



