162' G. A. J. COLE ON HOLLOW SPHEEULITES AND THEIR 



21. On HOLLOW SPHEErLiTES and their Occtjeeence in ancient 

 Beitish Lavas. By Geenyille A. J. Cole, Esq., P.G.S. (Eead 

 March 11, 1885.) 



[Plate IY.] 



It is now quite unnecessary to caU attention to the fact that many 

 of the Silurian '• felstones " of jS'orth Wales were originally lavas of 

 a highly glassy type. The investigation of their characters by Mr. 

 Eutley * and Prof. Bonney t, and their comparison with volcanic 

 rocks of far later periods, has fairly removed them from the con- 

 venient class of felstones, and given them a stricter position as 

 altered rhyolites, trachytes, or obsidians. Perlitic, spherulitic, 

 and banded structures have been recorded in them ; the present 

 paper deals with a structure which has been often noticed, and 

 which is far more obvious to the eye, its original character being, 

 however, veiled by much subsequent alteration. 



Every one who has examined the group of Bala lava-flows in the 

 Pass of Llanberis, notably at the foot of Esgair-felen, the great spur 

 of the Glyder-fawr J, or on the slopes leading to Cwm-glas, must 

 have observed whole masses of the compact grey rock crowded with 

 white or brownish spheroids. These are especially conspicuous on 

 weathered surfaces, or where a great joint-plane has made a section 

 through some hundreds, their diameters ranging from less than a 

 tenth of an inch to fullj^ two and a half inches. Some are evidently 

 radial aggregates, whether concretionary or amygdaloidal it would 

 be hard at first to say ; but others, and by far the most striking, are 

 mere hollow nodules, giving the rock a coarsely vesicular aspect. 

 Clear quartz- crystals often line the inner surfaces, and it is note- 

 worthy that the matrix has frequently the schistose character so 

 fully described by Prof. Bonney in the case of felsites near Bettws- 

 y-Coed §. This maj^ admit of considerable percolation of water and 

 the deposition of minerals in cavities already formed. That these 

 cavities are not, however, produced by indiscriminate weathering of 

 the matrix, but have some connexion with the original condition of 

 the lava-stream, is in places shown by the sharp demarcation 

 between a nodular bed and that immediately succeeding it. 



The solid spheroids, as a rule, seem composed of dense felsitic 

 matter ; if, however, they are not the result of the process of in- 

 filling that is in progress in the hollow nodules, the passage from 

 one type to the other is so gradual that a complete explanation of 

 the structure should cover both extremes. 



Before entering into any details with regard to these ancient 

 examples, it may be well to look for similar structures among com- 



* " On Perlitic and Spherulitic Structures in the La"vas of the Glyder 

 Fawr," Quart. Journ. Geol. Soc. vol. xxxv. p. 508 ; " On Devitrified Eocks 

 from Beddgelert and Snowdon," Quart. Journ, Geol. Soc. vol. xxxvii. p. 403. 



t "On some nodular Felsites in the Bala Group of North Wales," Quart. 

 Journ. Geol. Soc. vol. xxx^iii. p. 289. 



\ See Geological Survey of England and Wales, Horizontal Sections, Sheet 31. 



§ Quart. Journ. Geol. Soc. vol. xxx^iii. p. 291. 



