582 MINERALS AND VEINS OF iEGIRINE-SYENITE, 



ii. Classified Descriptions of the Veins. 



(a) Bitumen veins. — In the neighbourhood of Loveridge's main 

 quarry, traversing the carbonated syenite usually in an almost 

 vertical direction, are occasional black lines hardly broader than 

 a thread of cotton. Ready fracture of the rock along them dis- 

 closes considerable lateral extension. The main strings are 

 always plane and straight. In some instances several parallel 

 were noted crowded into a breadth of only a few centimetres. 

 Blindly ending branches often diverge for short distances. The 

 dark filling is pitch-like in appearance, and will burn on platinum 

 foil with a smoky flame leaving only a trace of ash. This forms 

 a very interesting occurrence of a hydrocarbon evidently distilled 

 from the Per mo-Carboniferous coal measures below and condensed 

 in fissures of escape. 



Examined microscopically, the bitumen is often seen to line 

 either side of the crack, whilst the central filling may be of 

 chalcedony. The rock traversed is completely carbonated, no 

 iron silicates or iron oxides showing; in one instance bitumen 

 itself was observed partly occupying the site of a former alkali- 

 iron silicate. 



(b) Simple pegmatite veins. — (1) Those of small dimensions, 

 width about 1 cm., also typified by absence or paucity of amphi- 

 bole. 



(a) Veins almost exclusively occupied by sanidine crystals. 

 These are irregular in direction, usually more or less horizontal, 

 extend only short distances, and preserve a subparallelism of the 

 sides. Where they are narrow, glassy felspar crystals interlace 

 across the whole width, leaving, however, much unoccupied space 

 between the individuals, In the case of wider fissures, drusy 

 walls have been noted. Alkali-iron minerals are absent or oidy 

 very sparingly represented. The probable explanation of this 

 type of vein formation seems to be that of contraction fissures of 

 limited extent, into which material capable of developing sanidine 

 crystals has sweated from the sides. 



