59S MINERALS AND VEINS OF yEGIRINE-SYENITE, 



normally only very little silica remains. A little calcite is 

 always present and invariably small amounts of fluorspar, 

 rendered readily perceptible by its violet tinge. The main body 

 of the paramorph is siderite often in well-shaped rhombohedrons, 

 more usually massive with a rough arrangement in the direction 

 of the amphibole cleavage 



The small quantities of fluorine present in the scattered grains 

 of fluorspar so generally associated with the amphibole decompo- 

 sition products may have been introduced with the carbon dioxide 

 at the time of carbonation, or may have had a primary origin 

 locked up in the amphibole until liberated by decomposition; the 

 detection of fluorine by analysis in the unaltered mineral is proof 

 that this latter explanation is correct, at least in part. 



Special features. — Slight variations from the typical arfved- 

 sonite are common. The border-zone often shades off into a 

 deeper blue variety which, though more opaque, has not the same 

 intensity of absorption-variation. 



This seems to be a parallel case with the zoned arrangement 

 more general amongst the segirine individuals, and indicating a 

 gradual change in chemical composition. Several cases were 

 noted where the amphibole was observed of this deeper colour 

 in proximity to minute penetrating fissures filled by secondary 

 silica depositions; in such cases, the chemical character of the 

 amphibole accessible for attack seems to have suffered change by 

 contact with vapours or solutions circulating after crystallisation 

 — a slow replacement of some of its molecules in the solid, as is 

 known to be readily possible in the light of modern chemical 

 knowledge. 



The central lighter portions are evidently more prone to decom- 

 position, as it is there, always, that carbonation commences (see 

 PI. li., tig. 4); when far advanced, the resistant blue border alone 

 remains enveloping a mass of carbonates. 



The amphibole derived from the uralitisation of the segirine 

 shows considerable variation; occasionally lighter-coloured with 

 marked absorption, closely resembling in its optical properties 

 Griqualand crocidolite; at other times it appears dense indigo- 



