Philip Lake — Bala Lake and River System. 211 



On the south-east side of the valley, these slates are visible close 

 to the Bala fault at Cilgellan, and again between Cefn-gwyn and 

 Cwm-onen. They, as well as all the other rocks, are well exposed 

 in and near Afou Dwrnudo.' They occur also in the banks of 

 a small sti-eam about 300 yards below Bryn Amlwg ; and again, 

 close to the Bryn Amlwg patch of volcanics, in the streamlets near 

 the transverse fault. 



On the other side of the valley, the slates are found in the farm- 

 yard of Pant-gwyn. They are well exposed in the triangular patch 

 four or five hundred yards north-west of that fai'm ; and in the 

 Dyfrdwy they are seen to overlie the volcanic series, dipping 

 5° S. of E. (true) at an angle of 38°. They occur also in the quarry 

 near Ty-mawr and in the bed of the river close to Tal-y-bont. 



In the neighbourhood of Llwyn-gwern these slates are visible in 

 several places, and the volcanic series dips beneath them. South- 

 east of Llanuwchllyn also, in the valley of Afon Twrch, the slates 

 and ashes ai-e clearly shown, and the ashes dip beneath the slates ; 

 but the boundary is here more complex than is represented upon 

 the map, being faulted by a series of little faults of which only one 

 or two are indicated. 



The north-to-south fault which lies east of Drws-y-nant-uchaf 

 is plainly visible in the stream 300 yards above the farm, where it 

 brings black slates (presumably Upper Lingula) on its eastern side 

 into contact with Middle Lingula Flags (containing Lingtilella Davisi) 

 on its western side. The eastern is therefore the downthrow side, 

 and as the hade is towards the west the fault is reversed. The 

 ci'ushing due to this fault is also seen in the little rivulet some 

 200 yards north-east of the farm. 



The fault which is drawn between Moel Caws and Creigiau 

 Llwyn-gwern nowhere, so far as is visible, brings beds of different 

 character into contact with each other, and is inserted upon less 

 evidence than the rest. But I venture to think that few who 

 examine the remarkable little valley in which it lies will be disposed 

 to deny the existence of this fault. 



The evidence for the other faults is manifest upon the raa2), and 

 need not here be recapitulated. In most cases the position and 

 direction can be fixed with considerable accuracy, but seldom so 

 precisely as to determine which of two intei'secting faults displaces 

 the other. It may be noted that the fault which crosses the Dwrnudo 

 600 yards above Llechwedd-fvvyalchen is probably continued for 

 a considerable distance in both directions. 



One or two patches of intrusive rock exist besides that which is 

 shown upon the map. Granophyre occurs on both sides of the 

 Bala fault at several points north-east of Drws-y-nant-uchaf, but is 

 too imperfectly exposed to be satisfactorily mapped. There are also 

 some masses of intrusive rock on the left bank of the Dyfrdwy, 

 near Rhyd-y-drain ; but these have not yet been examined in detail. 



^ Afon Dwrmulo is the stream wliicli flows past Llechwedd-fwyalcheu. 



