Vol. 55.] 



GEOLOGY OF NOKTHEKN ASTGLESEY. 



645 



these bands suggests again that we are near the overturned base of 

 the Ordovician. Beds of very similar character occur in the black 

 shales about | mile farther on, and also on the eastern slopes of 

 Mynydd-y-Garn. 



It may be remarked that along most of the course of the fault 

 the green beds are generally crumpled and crushed, the neighbour- 

 ing black shales are frequently similarly affected, and small outcrops 

 of white quartzite, evidently introduced by the movement, occasionally 

 mark its track. 



At Caerau the fault splits ; the more northerly branch brings in 

 a V-shaped Ordovician outcrop north of Mynachdy, the north- 

 westerly emerges at Forth yr Ebol. The former runs from Caerau 

 through Mynachdy Lodge to Forth I^ewydd. For the first part of 



Fig, 1. — Eastern side of Forth yr Ebol. 



AB, CD: 



: Intersecting thrusts. 1 = Green flaggy slates, much crushed and 

 brecciatecl. 2 = Ordovician blue-black slates. 



its course the Green Series, represented here by a purple slate, is 

 brought against the granite and altered rocks of Mynachdy, the 

 characteristic bed along the junction being a white quartzite, but 

 mingled with it are patches of limestone, grit, blue and dark 

 -quartzite, and black shale. Farther north the Ordovician comes on, 

 and the junction, though invisible, is unmistakably faulted. The dip 

 and strike of the two series are discordant ; on the north side the 

 beds are shattered by hundreds of tiny horizontal faults, and on 

 the south the black shales are highly compressed, and courses of 

 sandstone in them are squeezed into lenticles. 



The more westerly branch of the fault can be traced without 

 <difiiculty from near Mynachdy to Forth yr Ebol, where it may be 

 seen as an irregular synclinal curve in the cliff that forms the eastern 

 fiide of the Forth (fig. 1). This undulating ] unction has caused it 



