Vol. 54.] 



OF B0T7LAY BAY (jeESEt). 



113 



diameter, and, so far as the eye can see, quite identical with others 

 from the more immediate neighbourhood, is of a totally different type. 

 In a thin section the suggestion of a network which characterized the 

 matrix is replaced by a much more uniform appearance, owing to 

 the great development of indefinite films and specks rendering the 

 slide rather opaque — in large part due, perhaps, to the presence of 

 a chloritic mineral. There are some shadowy felspar-mieroliths and 

 no trace of a radial structure. Between crossed nicols the field 

 breaks up into large irregular areas, each polarizing separately. In 

 fact it closely resembles the patchy type of the Boulay Bay rocks 

 before described. 



Slides have also been prepared of an egg-shaped nodule and the 

 adjacent matrix from Digoed, on the road from Bettws-y-coed to 

 Penmachno. In the hand-specimen the '^ matrix' appears slightly 

 bluer than does the nodule ; minute specks of a greenish mineral 

 are present which, in a thin section, are arranged in a manner sug- 

 gestive, but not strikingly so, of flow. As in the former instance, the 

 rock is perlitic, but the cracks are not outlined by a chloritic mineral, 



Fig. 4. — Diagram showincf the relation of a nodule to jiow-hrecciation^ 

 (From the Wrockwardine districts) 









W^^^MrMM^}^^^ 



[The dotted portion represents siliceous infiltration. 

 Length of part figured = about '16 inch.] 



as is so frequently the case. The nodule, on the other hand, shows 

 wavy and contorted flow-lines resembling the specimen figured by 

 Prof. Bonney ^ from near the Conway Ealls Inn. 



The chocolate-brown rocks of the Wrockwardine district resemble 

 strikingly some of those from Boulay Bay, and the likeness between 

 the nodules of the two localities is equally strong in thin sections. 



^ Quart. Journ. Geol. See. vol. xxxviii (1882) pi. x, fig. 4. 

 Q. J. G. S. No. 213. I 



