Y Foel Fras, Caernarvonshire. 137 



No good exposure of the junction between the granophyre and the 

 sheared andesitic ashes which succeed in outcrop in a south-east 

 direction has as yet been found. This is owing to the comparative 

 ease with which the latter beds have weathered — ^the weathering 

 being assisted by the vertical cleavage. In neither of the above- 

 mentioned streams is there any exposure — the solid geology of this 

 area being obscured by drift. This band of ashes outcrops parallel 

 to the general trend of the granophyre over a width of about 

 600 feet. It can be followed from the saddle between Llwydmor and 

 Llwydmor Bach across the Cwm yr Afon Goch and up the northern 

 slopes of Bera Mawr. The rock is hopelessly weathered, with 

 abundant felspars showing occasionally a kernel of unweathered 

 plagioclase. 



Continuing the traverse up the Cwm yr Afon Goch, the 

 sheared ashes are succeeded by massive lava flows composed of 

 andesite intercalated with sheared ashes of similar composition. 

 Over these the stream rushes in a series of cascades, whilst in 

 two places the stream has cut. back its bed so that it now runs 

 through a miniature caiion 30 feet deep and only 5 to 10 feet wide 

 at the top. The andesite is usually of a dark-grey colour 

 with conspicuous and numerous plagioclase phenocrysts. It 

 weathers in the first stage to a greenish rock in which the felspars 

 appear white. At a further stage — well seen in the exposed 

 summits of Bera Mawr and Bera Bach — the weathering agency has 

 selected the felspar phenocrysts for destruction, so that the 

 surface presents a rough vesicular appearance and is quite pale- 

 grey with a lilac tinge. Owing to their proneness to weathering, 

 the vesicular selvages of the individual flows are rarely exposed. 

 The edge of the first flow is exposed in the crags on the left bank of 

 the Afon Goch just above the sheared ashes. Evidence from 

 the orientation of the felspar phenocrysts and from measurements 

 of the vesicular selvages of the flows, points to the fact that the 

 strike of these lavas is S.S.W.-N.N.E., and the dip about 90°. 

 These flows, with few intercalations of ashes, outcrop for about 

 1 mile in the direction of dip before they are succeeded by another 

 andesitic series in which ashes predominate over flows. Many of 

 these ashes are as massive and resistant to weathering as the massive 

 flows which form the rugged summits of Bera Mawr, Bera Bach, 

 and Llwydmor. They are coarser than the sheared ashes, described 

 above, the fragments of scoria occasionally measure 3 inches in 

 diameter. They form the upper north-west slopes and summit 

 of Foel Fras. Near the summit the effects of pressure again become 

 evident, as these beds when traced down the opposite south-east 

 flank become gradually more sheared. 



The massive andesites can be traced in the direction of their 

 strike, a distance of over 2 miles from the western shore of Lake 

 Anafon over Llwydmor and Bera Mawr to Bera Bach. It has not 

 been found possible up to the present to distinguish the individual 



