BY VV. G. WOOLNOUGH. 509 



decomposition does not appear to affect the strength of the double 

 refraction. As the process of alteration proceeds, the mineral 

 splits up into fibres whose refractive index and double refraction 

 are lower than those of the undecomposed substance. The inter- 

 stices between the fibres are iilled with a very weakly doubly 

 refracting substance of yellow colour. The final stage of decom- 

 position results in the production of aggregates of yellow-green 

 or greenish-blue chlorite. Sometimes these aggregates are 

 spherical-radial on a very small scale, each little sphere giving a 

 cross between crossed nicols; in other cases the aggregates are 

 homogeneous. The refractive index is low, and the double refrac- 

 tion exceedingly weak, much less than for apatite. The formation 

 of this chloritic material is accompanied by the separation of 

 grains, scales, and tufts of a white opaque mineral very suggestive 

 of leucoxene. The presence of considerable quantities of other 

 titanium minerals renders it by no means improbable that this is 

 the nature of the white mineral. 



Biotite is not recognisable. If it was present originally it has 

 been entirely converted into chlorite. Some of the aggregates of 

 the latter mineral are very fibrous, and suggestive of pseudo- 

 morphs after biotite; but, as all stages from these fibrous aggre- 

 gates to almost undecomposed hornblende are met with, it is safer 

 to refer all the chlorite to that source. In five slices of the rock 

 not one recognisable piece of biotite was met with. 



The most abundant and most important accessory mineral is 

 sphene. It is present mostly in the form of irregular grains of 

 yellowish-grey colour without pleochroism. A few of the sections 

 show some approach to the lozenge-shape, and in these the unsym- 

 metrical cleavage is well marked. The mineral is undoubtedly 

 of primary origin. 



The iron ore is magnetite, probably titaniferous. Where the 

 sections are idiomorphic, they show the forms characteristic of 

 magnetite. By reflected light, however, they are not so bright 

 as magnetite usually is, and are associated with grey and brownish 

 decomposition products. 



