296 VOLCANIC ROCKS ABOUT SNOIVDON. 



beds are traced in the vale of St. John, in Matterdale, in Eycott Hill 

 between Ulleswater and Haweswater, above Shap, and in other 

 places. 1 



Volcanic Rocks of Bala Age. The nodular felsites in the Bala 

 rocks of North Wales are well exhibited in the valley of the Conway 

 and Bett \vs-y-Coed, and have been studied by Professor Bonney near 

 the Conway Falls Inn. On both sides of the house the rock is a 

 compact felsite. It is succeeded by a bed with wavy lamina and 

 films of a green mineral, soon passing into more or less fissile and 

 coarsely spheroidal rock, with nodules as large as a pigeon's egg, 

 which, when weathered, give the rock the aspect of a conglomerate. 

 Other sections are seen by the wicket-gate leading to Conway Falls 

 from the road to Pandy Mill. The typical felsite is a compact bluish- 

 grey rock, which shows corrugated structure under the microscope, 

 due to the arrangement of microliths ; and nearly all the specimens 

 exhibit fluxion structure. The rock, which has a schistose character, 

 contains vesicles, which have become infiltrated with crystalline 

 < quartz, and with limonite, so that the rock is identified as a vesicular 

 rhyolitic lava. The nodular spherulitic rock shows no trace of a 

 radial structure, but closely resembles the ordinary felsites of the 

 neighbourhood. In the upper part of Conway Mountain, the yel- 

 lowish felsite contains spheroids, sometimes two inches in diameter, and 

 frequently hollow in the centre. This felsite is also seen in the 

 Diganwy Hills, where the ovoid masses have a cherty aspect, but 

 under the microscope they show fluxion structure like the matrix of 

 the rock. The ordinary cream-coloured felsite of the Conway moun- 

 tain has almost the aspect of a bedded mudstone. Professor Bonney 

 accounts for the formation of the nodules by contraction of the vesi- 

 cular rock, determined by the presence of a cavity, with roughly con- 

 centric cracking of the mass in cooling. 2 



Felstone of the Glyders. In a lava-flow from the Bala beds 

 forming the Glyders, on the north side of the Pass of Llanberis, Mr. 

 Frank Kutley has described perlitic and spherulitic structure. The 

 rock now termed felstone was originally a vitreous lava of the kind 

 named rhyolite, and the felstone character is entirely due to devitrifi- 

 cation. 3 



On the south side of the Capel Curig road, near Bedd-Gelert, is a 

 greenish-grey rock, which shows the usual characters of a de vitrified 

 rhyolitic lava, with spherules one-eighth of an inch in diameter, ar- 

 ranged in bands. The spherules are an aggregate of minute colourless 

 granules, probably garnets, and pale-green scales, probably of chlorite. 

 The rock was originally a pitchstone or obsidian. 



Another rock of a greenish-grey colour, which presents a banded 



1 Nicholson : "Greenslates and Porphyries of the Lake District," Q. J. G. S., 

 vol. xxvii. p. 599. 



2 Bonney : "Eelsites in the Bala Group of North Wales," Q. J. G. S., vol. 

 xxxviii. p. 280. 



a Rutley : "On Perlitic and Spherulitic Structures in the Lavas of the Glyder 

 Fawr," Q. J. G. S., vol. xxxv. p. 508, 



