Vol. 62.] IGNEOUS AND SEDIMENTARY ROCKS OP LLANGYNOG. 237 



(a) The llhyolites. — The rhyolitic rocks are best exposed in 

 the road immediately to the east of Capel Bethesda, where they 

 -dip a little north of west at 70°. They are tough blue-grey rocks 

 which weather white, and contain small but conspicuous cubes of 

 pyrites : occasionally they are markedly spherulitic. They are 

 bounded on the north by a fault, which separates them from the 

 Tetragraptus-gvits ; and on the south, although the ground is much 

 obscured by drift, there is every reason to think that they are 

 again faulted off from the main mass of Arenig sediments. The 

 estimated thickness of rock seen is certainly more than 400 feet : 

 and if, as seems probable, these rhyclites extend westward under 

 the conglomerate in the roadside-quarry mentioned on p. 231, then 

 the thickness would have to be greatly increased. 



(6) The Diabase. — At the cross-roads north of Pen-gelli- 

 uehaf , in an angle between two of the roads, is an old disused quarry 

 opened up in a very rotten, basic, igneous rock. The mass is full of 

 joints and cracks, and veined with calcite. On the surface of the 

 road to the east of the quarry the rock is well-exposed, but is quite 

 as rotten as before ; it has a bluish colour, and contains fairly-large 

 patches of calcite, which give it almost an amygdaloidal appearance. 



The road-section is about 60 yards in length, and is seen to 

 include a thin mass of sedimentary material in the form of greenish 

 indurated (? ashy) shales. These sediments may also be seen in the 

 northern hedgebank of the road to Glog, at 20 yards from the 

 cross- ways. Immediately north of these last-mentioned shales is a 

 steep furze-covered bank, in which by far the freshest diabase is 

 exposed. The intrusion is bounded on its northern side by grits 

 and shales of Tetragraptus-age, but the junction between the two 

 series of rocks is nowhere exposed. 



(3) The Lambstone Porphyry. 



The mass of igneous rock at Lambstone occupies a position due 

 west of the village of Llangynog [Llangunnock], and forms an 

 elliptical hill to the north of Lambstone Farm, nearly half a mile 

 long and one-sixth of a mile wide. The boundary between the 

 igneous rock and the surrounding sediments is marked by a strong 

 feature, which is easily traceable all round the hill. The mass is 

 faulted on its western and southern sides, but on the north and east 

 the junction seems to be a normal one. On the west it is faulted 

 against black shales containing graptolites of the Didymograptus- 

 bifidus type ; while, on the south, these give place to yellow, splintery, 

 and possibly-indurated shales belonging to the lower part of the 

 Tetragmptus-series. In the neighbourhood of Lambstone Farm, the 

 junction (which is still a fault) may be closely located, the shales 

 striking almost at right-angles to the igneous mass. 



On the eastern and northern sides of the hill the junction seems 

 to be an unfaulted one, for the shales strike parallel to the boundary 



Q. J. G. S. No. 246. s 



