584 MINERALS AND VEINS OF ^EGIRINE-SYENITE, 



On close inspection, these veins often show vestiges of a border 

 zone like that strongly developed in class (c). The texture is 

 very similar to that of the preceding subtype; a reticulating net- 

 work of idiomorphic sanidine crystals represents the product of 

 first deposition, arf vedsonite and a little eegirine occupy the inter- 

 spaces especially abundant towards the centre, conspicuous on 

 account of their contrasting colour. Quartz, perofskite, zircon 

 and ilmenite are also represented in the original crystallisation 

 but are very variable in amount, never more than accessories, 

 and often quite wanting. 



A second period of mineral deposition is evidenced, more or 

 less, in all cases examined. Original cavities, drusy with per- 

 fectly developed arfvedsonite tables, have been lined by chalce- 

 dony and filled, chiefly, by granular quartz; such areas occupied 

 by siliceous fillings are distributed most conspicuously down the 

 centre of the vein, testimony of the passage of mineralising 

 solutions. 



The silica of this second deposition is considered not to have 

 been derived from a like source as that producing the primary 

 crop of minerals. It has its origin beginning with decomposition 

 of original minerals by secondary active solutions; silica deposi- 

 tions in the trunk channels being a consequence of carbonation, 

 uralitisation, etc., elsewhere. As the classification implies, this 

 feature is here but slightly marked when comparison is made 

 with the next succeeding subclass described. 



(/3) Veins which have been subjected to powerful secondary 

 attack by active solutions. Incidentally, much oxide of iron has 

 been set free, pervading the formation with a general clove-brown 

 tint. 



In their earlier stages, these are identical with those described 

 immediately preceding; much that has been said therefore need 

 not lie further repeated. 



The very interesting phenomenon of their metamorphosis, in this 

 case best expressed by the term metachemisation, is the main 

 subject for description. The invasion of active solutions has 

 been chiefly into those veins of largest dimensions, having greatest 



