424 3IR. F. EUTLEY ON COMPOSITE SPHEKULI'lTS 



mitted light, is that of a portion of a large spherulite composed of 

 anastoniosing' flexuous rods or fibres diverging from an approximately 

 common centre. The rods or fibres are traversed transversely to 

 their length by delicate cracks, which are very numerous but 

 interrupted (fig. 2, PI. XVII.). Between the rods or fibres there is 

 more or less isotropic matter with minute globulites. The latter 

 o])pear to be ranged in irregular linear aggregates following the 

 direction of the crystalline rods. Trichites and granules and 

 minute crystals of magnetite are somewhat plentiful in this fibrous 

 mass. 



When viewed between crossed nicols the fibres or rods composing 

 the spherulite polarize, but not vividly, and the extinction, so far as 

 it can be made out in the matted mass of fibres, appears frequently 

 to be parallel and at right angles to their length, at other times it 

 seems to make an angle approximating to 21° with the long axis of 

 the rod, which would correspond to the extinction seen on (010) in 

 orthoclase. It would therefore appear that the divergent rods w hich 

 constitute the main hulk of the spherulite are orthoclase, as in the 

 cases described by Iddings in his work on " Obsidian Cliff." There 

 is, however, little dependence to be placed upon the measurement 

 of the extinction-angles, since the contused and branching character 

 of the rods (fig. 5, PI. XVII.) is, I believe, due to the fact that the 

 entire mass is made up of a succession of divergent bundles of 

 rods, similar to those met with in artificially devitrified glass. 

 Where this structure is strongly marked the section shows a well- 

 defined concentric banding (fig. 3, PI. XVII. ). each band occurring 

 at the terminal boundary of one zone of bundles of divergent rods, 

 which boundarj' forms the floor from which the divergent crystalline 

 fasciculi of the next zone grew. 



The most interesting point is that these concentric handings pa.9s 

 completely through the primitive spherulites already alluded to, as 

 shown in fig. 4, PI. XVII., which represents the same part of the 

 section given in fig. 8, the different appearances being due to the 

 methods of illumination respectively employed. It may therefore, 

 I think, be safely assumed that the divergent crystalline structure 

 of the large spherulite was developed subsequently to the massing 

 together of the small (primitive) ones, which are also traversed by 

 the divergent structure proper to the large or secondary spherulite. 



The small or primitive s])herulites show no fibrous or crystalline 

 structure of their own. What appears to be tridymite is present 

 in a small cavity situated at a little distance from the margin of the 

 s])herulite, and it is probable that some of the isotropic matter 

 Occurring between the divergent fibres of the spherulite may also be 

 referred to this mineral, as in some of the cases described by 

 Iddings *. 



Colourless microhths may be detected in the section, but they 

 do not appear to be numerous. 



8ome of the dark specks which, in reflected light, are seen to be 

 of a reddish or brownish coloui-, may be regarded as iron-o.\ides. 



* OjK cit. 



