186 ME. Gt. A. J. COLE 01^ THE ALTEKATION OF 



examples from the neighbourhood of Bettws-y-Coed. It is interesting 

 also to find that, before 1852, Delesse * examined at Jermyn Street 

 certain nodular specimens from Llanberis and Digoed f, and classed 

 them accuratel}^ with the '' globular porphyries " and pyromerides 

 of many foreign localities. The beautiful plates, drawn from 

 polished surfaces of large and altered spherulites, that accompany 

 his descriptions are monuments of faithful and minute work done 

 when section-grinding was unknown. 



In spite of their great size, the material of which the spherules 

 of Digoed are constructed appears to have been very finely granular, 

 and perhaps almost glassy. The relics of an often minutely curving 

 flow -structure J, and the occurrence of small crystals of felspar 

 scattered here and there, recall the features of the large spherulites 

 of Zwickau. But alteration has sometimes revealed an original 

 concentric structure, the secondary quartz occupying only the 

 alternate zones. In one of these cases there is beautiful evidence of 

 the pressure and strain § to which this rock has been subjected even 

 since the deposition of the secondary material. A series of parallel 

 bands of liquid-enclosures, containing moving bubbles, has been 

 developed in a curved zone of granular quartz ; these bands pass 

 continuously from grain to grain, and are reproduced on opposite 

 sides of the dull grey centre of the nodule, being thus clearly due to 

 one common cause, which has left no mark upon the felsitic zones 

 (PL IX. figs. 4 & 5). 



Ample signs of attempts to crush and cleave the rock are seen, 

 however, in the compression of the spherulites and sometimes in the 

 irregularly fissile character of the matrix, where it can be said to be 

 truly present. A cleavage has been also set up here and there in a 

 black alteration-product which occurs within the spherules, and 

 which, from its striking contrast to the hard, white, highly silicated 

 rock, has seemed to me to merit peculiar notice. 



Prof. Bonney, in the paper referred to ||, has mentioned a dull 

 flinty-looking, but soft substance as occurring among the secondary 

 minerals in felsites near Bettws-y-Coed and on the Conway Moun- 

 tain. In the rock of the latter locality the dark patches formed by 

 it are unusually abundant, and their relation to the zones of alteration 

 in the nodules is often beautifully seen. One flattened spherulite, 

 measuring 18 millimetres in its longest diameter, consists of at least 

 thirty concentric coats, half of them being converted into the black 

 material. The wavy structure seen in the latter, under the micro- 

 scope, seems, from its general persistence of direction in the diff'erent 

 zones, to be due to an attempt at cleavage which has been resisted 

 by the less yielding portions of the rock (PI. IX. fig. 6). 



At Digoed this dark substance occurs on a coarser scale, so as to 



* "Eecherches sur les Roches globuleuses." Memoires de la Soc, geol. de 

 France, 2me serie, tome iv. (1852), pp. 315 and 327. 



t These specimens may be seen in the rock-room of the Greological Survey 

 Museum, numbered 382, &c., in the 3rd edition of the Catalogue. 



+ Cf. Bonney, Q. J. G. S. vol. xxxviii. p. 294. 



§ See Judd, Q. J. G. S. vol. xli. p. 376. 



II Q. J. a. S. vol. xxxviii. pp. 290, 293. 



