110 MR. J. PARKINSON 01^ THE PYROMERIDES [Ycb. 1 898, 



It seems then fairly evident that, taking this rock as a whole, 

 two materials were present in the magma. One, the more flnid, has 

 solidified as a glass, now represented by the yellowish-green 

 ' matrix ;' the other, which was in a sticky state, has been drawn out 

 into irregular flow-bands, or left as lumps and clots. That it was 

 occasionally melted is shown by the fact that flow-lines may 

 in places be seen traversing uninterruptedly both pyromeride and 

 ' matrix.' On the cessation of movement in the lava, and as the 

 temperature fell, the radial structure of the pyromeride would be 

 set up when this was possible. In one case the outer part of one 

 of the purplish masses has distinctly more of the structure of a 

 pyromeride, in a well-defined zone, than the more central, which has 

 indications of flow. 



Nevertheless, definite spheroidal forms do occur, which have 

 strongly- marked characteristics of their own, resembling, in these 

 and in general relations those recorded above, which appear to have 

 originated each at a separate point, like true spherulitic individuals, 

 from a magma homogeneous at the initial stage of their formation. 

 Such a rock as that described and analysed by Prof. Bonney and 

 Mr. Hyndman forms an example. 



Among those rocks which show evidence of flow, the relation of 

 which to the pyromerides has just been discussed, occur patches 

 about the size of one's hand, in which a network of polygonal cracks 

 is conspicuous. These, often irregular, are frequently developed 

 round darker and roughly circular spots showing the structure of 

 pyromerides ; the feeble radial structure is not, however, confined to 

 this more central and definite part, but is continued throughout the 

 space which any one set of the cracks encloses. 



Crescents of quartz are found occasionally towards the outer edge 

 of the darker centre.^ In general, there is a decided resemblance to 

 the spherulitic felsite figured by Prof. Bonney from Arran. 



III. Comparison with other Districts. 



Structures similar to many of those from Boulay Bay which have 

 been described above have been recorded in a very valuable work 

 by M. Delesse so far back as 1852. 



The following remarks will scarcely bear condensation. In 

 describing some spheroids (from Siberia), after remarking that they 

 are usually irregular, he says : — ' Tantot ils sent completement 

 i soles ; tantot ils sont accoles a des bandes jaspees qui leur servent 

 de parois, et il semble meme que le globule resulte de la reunion en 

 spheres de ces bandes jaspees.' " Apart from external form, Delesse 

 also clearly understood that contraction during cooling had had 

 much to do with the structure of pyromerides ^ ; distinguishing be- 

 tween ' des cavites petites et assez regulieres ' produced by such an 



^ ' Pitclistones & Felsites in Arran,' Geol. Mag. 1877, p. 499. 

 ^ Mem. Soc. geol. France, ser. 2, vol. iv (1852) pt. ii, p. 316. 

 3 Ibid. p. 336. 



