232 



MESSES. T. C. CANTEILL AND H. H. THOMAS ON THE [May I Qo6 r 



beds of the Old Red Sandstone or to the Tetragraptus-Griits : the 

 coarseness of the material, however, compared with that of the 

 Tetragraptus- conglomerate of Glog-ddu Hill (500 yards to the 

 south-east) has alone determined us to group it provisionally with 

 the Old Red Sandstone. Some 50 or 60 yards north-west of the 



quarry, and on the top of the ridge 

 towards its western end, one or two 

 angular masses of white -weathering 

 rhyolite project through the soil. These 

 do not appear to belong to the conglo- 

 merate ; and if they are not blocks, 

 ice-borne from the exposure of that rock 

 at the Chapel, they must be the top 

 of a rhyolite-mass projecting through 

 the conglomerate. The rhyolite at 

 the Chapel is striated west 22° south, a 

 direction which points towards the 

 possible boulders. 



IV. The Igneous Rocks. 



The igneous rocks of the Llangynog 

 district occur in three well-defined areas 

 which, taken according to their im- 

 portance, are (1) near Coomb ; (2) at 

 Capel Bethesda ; and (3) at Lambstone. 



(1) The Rocks near Coomb. 



The igneous rocks in the neighbour- 

 hood of Coomb occupy an area of about 

 half a square mile, and are bounded on 

 the south and east by the Old Red 

 Sandstone, which unconformably over- 

 lies them ; while on the north and west 

 they are faulted against Tetragraptus- 

 Beds and Didymograptus-bifidus Beds. 

 This area of igneous rocks is naturally 

 divided into two unequal portions by a 

 dingle through which a stream flows 

 southward from near the Plough-&- 

 Harrow Inn, past Llwyn-celyn, and 

 ultimately falls into the estuary of the Taf. This dingle will 

 hereafter be referred to as ' the Coomb Dingle/ 



On the eastern side of the dingle, the rocks consist of two series 

 of andesites and associated tuffs, separated by a mass of rhyolites 

 and rhyolitic breccia. On the western side, to the north, occurs 

 a series of andesites into which the diabase of Tre-hyrn has been 

 intruded, and to the south lies the large mass of rhyolitic rocks of 

 Castell Cogan. A small patch of intrusive diabase appears at the 

 extreme south-west of the area, near Pentre-newydd. 



