ON DEVITKIFIED ROCKS FROM BEDDG-ELERT AND SNOWDON. 403 



31. On the Microscopic Structure of Devitrified Rocks from 

 Beddgelert and Snowdon ; with an Appendix on the Eruptive 

 Rocks of Skomer Island. By Frank Rutlet, Esq., F.G.S. 

 (Read April 6, 1881.) 



[Plate XXL] 



The specimen from the south side of the Capel Curig road, about | mile 

 from Beddgelert, which was collected by Mr. George G. Butler *, and 

 which he kindly placed at my disposal, is a rather dark greenish- 

 grey rock, spotted with pale greenish-grey spherules, some of which 

 are over i inch in diameter, and which, besides occurring isolated 

 and in approximately spherical forms, have also coalesced, forming 

 bands from 1 inch to nearly \ inch in breadth. On a smoothly cut 

 surface they afford, by their pale tint, a strong contrast to the dark 

 matrix. The isolated spherules and spherulitic bands are shown in 

 fig. 3 (PI. XXI.) as they appear when magnified about 6 diameters. 

 A thin section, when examined between crossed Xicols, is seen to 

 be studded all over with small doubly refracting specks. By ordi- 

 nary transmitted light, and under a power of 25 diameters, a marked 

 difference is visible in the microscopic characters of the spherules 

 and the matrix, the former being almost colourless, save for a few 

 pale greenish flecks, which become more closely aggregated at the 

 margins of the spherules, forming a somewhat darker border, while 

 the matrix appears to consist of a closely matted or granulated deep- 

 green substance resembling chlorite, through which are interspersed 

 a great number of clear spots consisting of nearly colourless matter, 

 similar to that which composes the spherules. Between the sphe- 

 rules and the matrix, forming a sharp boundary, is a clear, narrow, 

 colourless border (fig. 1, PI. XXL), while the matrix itself is tra- 

 versed by more or less sharply defined hues, which also appear clear 

 and colourless. 



These lines seem, in places, to be nearly straight, and to divide 

 * Extract from letter from G. G. Butler, Esq. : — 



" With regard to the fragment of spherulitic felstone, I have only to say that 



1 knocked it off the corner a piece of ;rock, perhaps 6 or 8 feet square and 



2 or 3 high, projecting from t a sloping field on the south side of the road from 

 Beddgelert to Capel Curig — about a quarter of a mile from the former place, and 

 perhaps 200 yards from the road, the field sloping down towards the road. 

 The rock appeared to be in situ. I found similar specimens on other protruding 

 rocks near, but none so perfect as this. I fear I cannot give any more informa- 

 tion about it. 



"A rock which abuts on the north side of the road, nearly opposite, but not 

 so far from Beddgelert (in fact, just at the edge of the village), presents a curious 

 appearance from a number of globular bodies contained in it averaging 2 or 3 

 inches in diameter and standing half out from its surface. When knocked in a 

 careful way, they come out bodily, leaving an empty hemispherical socket. 



" I remain, very truly yours, 



"Gr. G. Butler." 



