DEVITRIFIED ROCKS EROM BEDDGELERT AND SNWDON. 405 



identical in appearance with the little grains in the Beddgelert 

 rock. 



From the general character of this rock, from its spherules and 

 spherulitic bands, and from the vestiges of perlitic structure which 

 it presents, I have no hesitation in regarding it as a devitrified obsi- 

 dian or pitchstone. This might, however, be inferred by any practised 

 geologist without recourse to the microscope ; and it is only right to 

 add that Mr. Butler was well aware of its nature when he gave me 

 the specimen. 



The rock next to be described is associated with Bala beds, and 

 occurs at Clogwyn d'ur Arddu, a high ridge about 1 mile N.W. of 

 the summit of Snowdon. The specimen from which the section has 

 been cut was collected long ago by Professor Ramsay*, and is now 

 in the rock- collection in Jermyn Street. 



The specimen is of a greenish grey colour, and shows an interest- 

 ing weathered surface with projecting bands lying closely together, 

 and separated by rather deep and narrow furrows. Speaking of 

 these bands, Prof. Ramsay states that they " probably originated in 

 the same cause that produced the lamination in the lava of Ascension"f . 

 Under the microscope (fig. 2, PL XXI.) the section exhibits an irre- 

 gular wavy-banded structure, such as might have been inferred from 

 the banded character of the weathered surface. In polarized light this 

 banding is marked by a strong difference in texture or grain ; for the 

 rock throughout is micro crystalline and is now feist one. 



A comparison of this rock with unaltered banded obsidians of com- 

 paratively late geological age, such as those of Ascension, the Liparis, 

 the Yellowstone, and other volcanic districts, leaves but little doubt 

 that the structure has resulted from fluxion ; and I think we may 

 also assume that the rock was once vitreous. 



On the right-hand side of the road, between Pont-y-Gromlech 

 and Gorphwysfa, as we ascend the Llanberis Pass, an outcrop of 

 dark grey felstone-like rock occurs, which breaks or splinters under 

 the hammer into irregular slabs or platy fragments. The fissile 

 structure appears at first sight to be due to the presence of dark 

 greenish-black films with an oily lustre, resembling patches of 

 talcose slate. It is, however, possible that other and more minute 

 structure may also have some share in imparting this schistose cha- 

 racter to the rock. In its present condition it may be termed a 

 felsite schist ; and it is probable that many geologists would, from 

 the general appearance of hand-specimens, regard it as an indurated 

 volcanic ash. For a long time the microscopic character of this 

 rock has been a source of perplexity to me. tinder the microscope, 

 by ordinary transmitted light, the section is seen to be made up of 

 small fragments, strings, and shreds of every shape, separated by 

 finely granular and less translucent matter, which is strongly im- 

 pregnated with a very pale greenish chloritic substance forming very 



* By permission of Prof. Ramsay this description is now laid before the 

 Society. 



t Descriptive Catalogue of Eock-specimens in the Museum of Practical Geo- 

 logy, 3rd edit. (1862), p. 42, spec. 374, wall-case 41. 

 Q. J. G. S. No. 147. 2 e 



