Y Foel Fras, Caernarvonshire. 135 



The investigation includes the horseshoe-shaped ring of mountains, 

 of which the highest is Y Foel Fras (3,091 feet), surrounding the 

 upper course of the Aber River (Cwm yr Afon Goch). To the 

 north-west this igneous complex is bounded by the precipitations 

 crags over which the above stream descends by leaps and bounds, 

 forming the well-known Aber Falls. Upon Sheet 78, south-east of the 

 old Geological Survey map the area is coloured as " Massive 

 intrusive felspathic rocks ", surrounding two oval patches indicated 

 as " Greenstones ", whilst a further elongated patch of the latter 

 forms the south-west boimdary. The north-east boundary is 

 limited by the Aber-Llanbedr fault. Besides the hog-backed 

 summit of Y Foel Fras, the area includes the similarly outlined 

 summits of Y Drosgl West^ (2,483 feet), Yr Arryg (2,875 feet), 

 Llwydmor Bach (2,256 feet), and the more rugged summits of 

 Bera Mawr (2,587 feet), Bera Bach and Llwydmor 1:2,749 feet). 



A study of this area commenced early in the summer of last 

 year shows that the map will require considerable modification. 

 The north-west boundary consists of a lenticular outcrop of augitic 

 granophyre, nearly 3 miles in length, from north-east to south-west, 

 and reaching a maximum width of about 1,200 yards at Y Drosgl 

 West. The microstructure of the rock has been described by 

 Dr. Harker ^ and an analysis published. The mass is undoubtedly 

 intrusive as Ramsay remarks,^ as for distances varying between about 

 30 and 100 feet from the contact the slates are baked to porcellanite. 

 It is evident, however, that Ramsay mistook the baked slate which 

 weathers yellow for the chilled edge of the intrusion. He mentions 

 that the intrusion edge " rises like a wall, is fine grained and weathers 

 yellow ". A careful examination of the junction at the falls of the 

 Afon Bach shows that it is not at all regular, but that the granophyre 

 sends veins into the baked sedimentaries. Also it is noticed that it 

 is the indurated baked slate that forms the walls of the precipice 

 in most places, and it is seen that this metamorphosed rock weathers 

 better than the intrusion. This is well shown on the summit of 

 Gyrn Wigau, where the only exposure is a well-defined ridge running 

 from north to south, composed of this dark-grey porcellanite, 

 weathering pale yellow, while the outcrop of the granophyre to the 

 east of the ridge can be deduced only from scattered blocks. 



The thermo-metamorphism of the contact rock is less pronounced 

 than would be anticipated from the dimensions of the intrusion. 

 A micro-slice cut from a specimen from the exposure at the lesser 

 Aber Falls * shows no distinct recrystallization except for minute 

 grains of magnetite which increase in number towards the junction. 



1 It is necessary to add the suffix "West " to distinguish this mountain 

 from the other of the same name 3 miles north-east, i.e. between Foel Fras 

 and Tal-y-fan. 



^ A. Harker, Bala Volcanic Series of Rocks, pp. 41-8. 



* Ramsey, Geological Survey Memoir N. Wales, pp. 138-9. 



* Lent by Dr. E. Greenly. 



