Vol. 54.] OF BOULAY BAY (jEKSEY). 109 



band of rock with flow-structure but no ' spheres.' Taken over 

 the whole area (which is not large, forming but a few crags on 

 the beach), the general direction of flow is maintained ; yet, taken 

 in detail, the appearance is much less distinct, and in many places 

 the various constituents have a very clotted look. 



The structures in a thin section of this rock, showing well-marked 

 purplish pyromerides set in a yellowish-green ' matrix/ are as 

 follows : — This ' matrix,' although slightly obscured by the pre- 

 sence of secondary decomposition-products, is still quite clear 

 enough to shovS' that this part of the rock (now microcrj^stalline) 

 exhibits perlitic structure on a fine scale, the cracks, as in other 

 cases, being outlined by a green mineral. 



In comparison with this ' matrix ' the pyromerides are almost 

 opaque. They have a wavy, almost scalloped boundary, and there 

 seems to be at least a suggestion that the shape of the edge of 

 a pyromeride has determined the shape and proximity of the neigh- 

 bouring perlitic fissures. The pyromerides themselves show indica- 

 tions of radial structure such as have been mentioned before, 

 together with the formation of more opaque patches, which, while 

 preserving a general radial direction, constitute a kind of irregular 

 meshwork more extensively^developed in some places than in others. 

 Often the periphery of the spheroids is even and comparatively 

 regular, but often too there is an irregular and clotted appearance, 

 with edges drawn out into short and blunt tongues (PI. VII, fig. 2) ; 

 these, practically destitute of any attempt at radial growth, succeed 

 to a more central part, which, although structurally a pyromeride, 

 obviouslj^ forms part of the same mass. 



This suggests the presence of material identical with that forming 

 the pyromerides, but unable, owing to local conditions, to adapt 

 itself to their characteristic mode of growth. It is interesting 

 therefore to note that there are frequently present in the rock 

 irregular patches of the same purplish constituent, which, although 

 found quite locally, may under these circumstances take on the 

 functions of the ' matrix.' 



In a thin section, as in the hand-specimen, this less specialized 

 material is at once distinguishable from the adjoining true matrix : 

 the latter, as before mentioned, being finely microcrystalline, and 

 containing some quantity of a greenish filmy mineral. 



On the other hand, the material in which the pyromerides are 

 partly embedded is slightly more opaque, more ferriferous, and, 

 between crossed nicols, the regular mosaic of grains shown by the 

 ' matrix ' proper is replaced by a more irregular patchy type of 

 depolarization, the boundaries of the constituent areas into which 

 it breaks up being ill defined. In addition, within the limits 

 of this material itself there are distinct evidences of flow. Although 

 the enclosed pyromerides are clearly distinguishable, yet it is 

 evident that the characters of the differentiated and undifferen- 

 tiated materials are nearly alike, and the intimate relation which 

 exists between them points inevitably to the conclusion that they 

 are indeed the same. 



I 



