ASSOCIATED ROCKS OF THE BEEIDDEN HILLS. 



535 



Fig. 1. — Section across Middletown Hill. (Length, \ mile.) 



C S'. Cambrian Shales, 

 V A. Volcanic ash. 

 CS", Cambrian Shales. 



Q,: 



YC" 

 : quarries. 



Lower Volcanic Conglomerate. 

 Upper Volcanic Conglomerate. 

 C. Little crags. //. Fault. 



kaolin and quartz. This ash is again seen to the S. of Moel-y- 

 Golfa, and in one or two isolated spots to the N.E. It passes up 

 into andesitic conglomerate, and that, in a quarry at the foot of 

 the hill, into 10' of volcanic grit without pebbles, followed by sand- 

 stone and shale with hard grit bands containing Pentamerus ; a 

 faidt obscures the exact relations of the two groups. 



The lowest beds seen in Moel-y-Golfa are lavas of an andesitic 

 type, well exposed in the great crags at the S.W. end of the hill. 

 Similar lavas are seen to succeed one another on the S.E. side, one 

 of the lowest of them being amygdaloidal, to a total thickness perhaps 

 of 400 feet, until, at about 650 feet from the summit, they are fol- 

 lowed by ashy beds and these by conglomerates, both of which wrap 

 round the S.W. end of the hill ; these are visible in the crags and 

 quarries on and near the Welshpool road. The ash, when not of 

 the china-stone type, is obviously formed of the same materials as the 

 lavas of the hill, and the conglomerates, in which I have measured 

 fragments 2 feet long, are composed of lumps of the same rock. 



The hill is very abrupt at its S.W. extremity, and the ashes and 

 conglomerates do not thin out as they do northwards, but are re- 

 placed on the plain by the shales of Trewern, like those found else- 

 where beneath the conglomerate, suggesting that the beds are cut 

 off by a fault; but there is no additional evidence of this unless we 

 consider the absence of the black grit of Belleisle Brook to be such. 



At Oefn, near Buttington, there is an inlier, probably of the same 

 rocks, associated with an intrusion of diabase. The quarries show 

 shivery grey shales interbedded with grits often baked to quartzite 

 and very much contorted by the intrusion *. 



Prom this description .it will be seen that the centre of volcanic 

 activity was at Moel-y-Golfa, where the lavas occur and near to 

 which the ash beds are thickest, while further north the area was 

 for the most part submerged, becoming occasionally shallow for the 

 formation of conglomerates and even at times upheaved to receive 

 * Silurian System, 292. 



