138 Normfian L. Silvester — Igneous Complex of 



ilows in the field by means of petrographie difierences. The summit 

 of Lwydmor as viewed from the valley of the Anafon below the lake 

 consists of a succession of five flows whose basset edges stand out 

 boldly against the skyline like the castellations on a castle wall, 

 whilst on Bera Mawr they number six. 



The proximity of the vent from which this andesitic series was 

 €xtruded seems clearly evident from the occurrence immediately 

 to the west of Bera Bach of a small outcrop of coarse agglomerate 

 or breccia consisting of large fragments of fine-grained vesicular 

 andesite measuring as much as 2 ft. 6 in. in length. The interstices 

 are filled with smaller fragments of ash, and the cementing material 

 is a soft, dark slate in which no fossils could be found. Exposed as 

 they are at a height of over 2,000 feet to the full force of the prevalent 

 winds, these agglomerates are deeply weathered to a uniform 

 pale grey, looking remarkably like weathered rhyolites in the 

 distance. As the cementing material has weathered faster than the 

 lava blocks, the latter stand out from the surface so as to resemble 

 the loose debris of a recent eruption. The dip of the longer axes 

 of most of the blocks is south-east at 70° to 80°. 



According to Dr. Harker ^ the whole of the area consists of the 

 plug of a vast volcano built on the site of and partly out of the 

 fragments of an older vent. This hypothesis readily removes 

 the difficulty of explaining the sudden disappearance of the series 

 when traced in the direction of the strike to the north-east. It 

 will also explain the vertical position of the lava flows if it is assumed 

 that the plug successfully resisted the later earth-movements 

 without being thrown out of the vertical. It is, however, not easy 

 to account for the persistence of the strike of the lava flows. If 

 they are merely the feeders which supplied the material for the ashes 

 and for flows since removed by denudation, it is to be expected that 

 their outcrop in cross section would be approximately circular 

 unless the eruption were of the fissure type. If, on the other hand, 

 this series represents the remains of a normal succession of flows 

 and intercalated ashes originally laid down horizontally but since 

 thrown on edge by crust movements, we have a succession nearly 

 2 miles thick comparable in magnitude to the Borrowdale series in 

 the Lake District. At the eastern extremity of the area, as in the 

 succession exposed in Pant y Mynach to the south of L. Anafon, 

 rhyolitic flows and ashes become intercalated with the andesitic 

 series. This rhyolitic series thickens north-eastward, but dies out 

 in the opposite direction between Foel Fras and Llwydmor. It 

 therefore appears that the vicinity of Llyn Anafon is an area inter- 

 mediate between two vents, one near Bera Bach pouring out andesitic 

 material simultaneously with another to the north-east ejecting 

 more acid material; Ashes formed of fragments of both kinds are 

 found on the west shore of Llyn Anafon. 



^ Harker, Bala Volcanic Series of Rocks, p. 127. 



