S02 ME. G. A. J. COLE OX SOME ADDITIOl^AL 



(PI. XI. fig. 2). At the same time the material of the spherulites, 

 settling into a crystallized condition, becomes greyish in certain 

 sectors and richly brown in others, A minute vesicle, filled with 

 secondary minerals, occurs frequently at the centre of the spheru- 

 lite, and may perhaps be a feature of alteration, extending inwards 

 from the point of weakness ; but, on the other hand, the regular 

 outline of these vesicles gives support to the view of several writers, 

 that such gas-bubbles may, like included bodies, serve as centres of 

 devitrification *. 



The radial structure in the Ardtun spherulites is seldom regular, 

 though clearly seen ; and the black cross, its arms parallel to the 

 vibration-planes of the crossed nicols, is frequently much disturbed. 

 "With a single nicol the spherulites, when well developed, show 

 remarkable phenomena of absorption. The browner fibres become 

 of a darker hue when their longer axes are placed parallel to the 

 shorter diagonal of the nicol ; while the greyer groups, in the same 

 spherulite, are at their darkest in the reverse position. Since this 

 change of hue is wevy striking, a dark bar like one half of the cross, 

 and due to brown or greyer fibres, is frequently seen traversing the 

 spherulite when only one nicol is employed : and, if grey and brown 

 sectors are suitably grouped in the same example, a complete dark 

 cross may be shadowed out in one position, rotation of the slide 

 through 90° reversing the conditions, and showing both vertically 

 and horizontally placed fibres at their lightest f. 



While I am not prepared to explain the pleochroism of the greyer 

 sectors, it seems probable that, among the fibres of these basic 

 spherulites, a separation into distinct mineral substances has oc- 

 curred J. The greater tendency to crystallization in basaltic rocks 

 may, indeed, cause a granophyric structure to arise under conditions 

 that would, in acid lavas, produce spherulites practically isotropic. 

 The surface of a section of the Ardtun rock, when etched with 

 hydrofluoric acid, gives no very definite evidence ; but somewhat 

 greater resistance to the acid is ofifered by the browner rays. It 

 must be borne in mind that the minerals composing such a grano- 

 phyric spherule — a " pseudospherulite " of Eosenbusch — may be far 

 different from those that separate during a less hurried process 

 of crystallization, and that combinations may indeed be formed 

 unknown to the cabinets of collectors. In augites rich in alkalies, 

 however, such as are developed in many glassy rocks, the axis of 

 maximum elasticity approximates to the vertical crystallographic 

 axis, and a prismatic section exhibits its darkest tint when the 



* The possible Uberation of gas during the formation of a spherulite is 

 discussed by Iddings (Amer. Journ. of Sci. vol. xxxiii. (1887), p. 43). 



t Such phenomena of absorption seem to be by no means rare. They occur, 

 for example, in a spberulitic tachvlj'te from Tasmania, and in the spherulites 

 developed in artificial basalt-glass, as in the fused product of the " Eowley 

 Eag." Even some acid lavas, as the perlitic rocks of Hlinik and of Puszti Hrad 

 in Hungary, contain spherulites with pleochroic rays. 



I I am glad to find that Dr, Wenjukoff holds the same opinion with regard 

 to the spherulitic tachylyte of Sichota Alin (BulL Soc. Beige de Geologic, 

 tome i. (1887), p. 174). . 



