Vol.51.] THE COUNTKT AEOTJND FISHGUAKD. IBS' 



The 'pale green oval spherulites, about | inch in length/ 

 described by Miss Raisin as occurring in the Deunant and Aberdaron 

 variolites, 1 bear a very close resemblance to these nodules, and 

 indeed show no important points of difference ; but I can see no 

 reason why the structures in question should be considered spheru- 

 litic, as they show little or no radial arrangement. 



I have had the advantage, through Miss Raisin's courtesy, of 

 examining her slides of these rocks ; and it appears to me that we 

 have to deal with structures similar to some described by her in the 

 nodular felstones of the Lleyn. The cracks which mark off the 

 ' nodules ' from the groundmass gain therefore, in my opinion, 

 considerable importance in the Penrhiw rock, since they indicate the 

 mode of origin of the nodules, namely, by contraction. Prof. Bonney, 

 who has seen my slides, makes the additional suggestive remark 

 that the groundmass may have shown an original ' lumpiness ' or 

 ' clotted ' character, indicating local differences of composition and 

 leading to the formation of the nodules. 



In two cases, [31] and [40], portions of the groundmass, which in 

 ordinary light are seen to be free from the greenish rod-like structures,, 

 but crowded with minute spherical granules, show with crossed 

 nicols numbers of secondary spherulites which give a black cross. 

 In some cases they appear to have been formed around small, 

 circular, granular brown spots, but in others these spots occur 

 promiscuously in them. The fine crystallites, 'etc., traverse the 

 spherulites indifferently and without deflection or alteration of any 

 kind, thus bearing testimony to the secondary origin of the spheru- 

 lites. In one slide [31] undoubtedly secondary spherulites occur in 

 a vesicle. 



The true amygdaloids of spherical shape, which may be compared 

 with those in the previously-described Caerlem rock [262], are 

 mostly composed of microcrystalline quartz, but some are of clear 

 crystalline quartz-grains. The groundmass, where not spherulitic, 

 has an almost isotropic or a blurred appearance with crossed nicols. 

 [224], [31], and [40]. 



A rock in close association with these ' nodular ' varieties shows 

 perlitic cracks with great distinctness [77]. None of the rod-like 

 crystallites and very few of the granules are present in this rock. 

 But the groundmass looks as if it were composed of dense multi- 

 tudes of overlapping, minute, brownish-yellow prisms with a high 

 index of refraction. Each portion of the groundmass enclosed by a 

 perlitic crack has this appearance in the central portion, but it is 

 surrounded by an outer zone of a pale brownish-yellow, homogeneous 

 substance. Prof. Bonney has called my attention to certain points 

 in the character of the perlitic cracks which are of especial interest. 

 Each crack is indicated now by a dark, thread-like line, on each side 

 of which lies a clear transparent broad line, and outside of this lies 

 the brownish-yellow homogeneous zone which has been mentioned 

 as surrounding the separate patches of the groundmass. Occasionally 



1 Quart. Journ. Geol. Soc. vol. xlix. (1893) p. 154. 

 Q. J. G. B. No. 202. p 



