Western Auiitralia. 157 



others, with the result that distinct lines of weakness were pioduced 

 in the rocks. 



At a later date, along these lines of weakness and doubtless 

 along various fissures in the mass, there were intrusions of a highlv 

 acid dj'ke-rock rich in soda and with frequent hornblende 

 phenocrysts — the albite— porphyrite or keratophyre. That this is 

 decidedly later in origin tlian the greenstones is proved by its 

 occurrence in tongues in them, and by the frequent examples in one 

 locality of included chloritic xenoliths. 



Accompanying the acid intrusions were boric vapours which not 

 only caused the production of tourmaline both in the keratophyre 

 and in the quartz leaders, but which, in association with other gases, 

 without doubt exerted a pneumatolytic action on the surrounding rocks. 

 Harker' has briefly described the main views now held as to tliis type 

 of alteration. According to him, with the completion of crystallization 

 in a rock magma the combined water and other volatile substances 

 must be disengaged. The bulk of these are collected in druses and 

 geodes or occupy fissures in the now solid rock body. Such cavities 

 and fissures may be sufficient to permit of a general circulatioTi 

 tlirough the rock mass. As cooling proceeds, however, some of the 

 compounds crystallized at higher temperatures cease to be stable in 

 the presence of the concentrated gaseous residuum and are decom- 

 posed with the production of new minerals. The volatile substances 

 may or may not enter into the composition of these new minerals, but 

 in general they do so at a greater extent at this stage than during^ 

 the magraatic crystallization. 



The pneumatolytic agents themselves are merely the final residuum 

 of an igneous rock magma, and the processes are conducted at 

 temperatures more or less elevated, though progressively declining. 

 It is probable, therefore, that the tourmaline of the keratophyre has 

 been produced during the later stage of the consolidation of the 

 magma and possibly by action of the boric vapours on some of the 

 felspar. 



Further, in the lode formations kaolin and sericite are particularly 

 abundant, and, as Flett has pointed out, this kaolinization and 

 sericitization may be attributed in great part to pneumatolytic action. 

 Moreover, the action of the vapours was doubtless also exerted on thr 

 surrounding greenstones, causing a propylitization which has given 

 rise to some at least of the soft greenish chloi-itic rocks. 



When the high acidity of the dyke is taken into consideration, and 

 also the fact that frequent cases occur of the quartz leaders enclosing 

 tourmaline needles, most of the leaders and of the auriferous quartz 

 of the lodes may be regarded as genetically connected with the dyke. 

 as being the later more acid or wholly acid phase in which the rarer 

 raineralizers — in this case boric vapours — were particularly abundant. 

 It is probable also that auriferous veins will be found not only at the 

 junction of the dyke with the greenstones, but in some cases for some 

 distance into the greenstones themselves. 



' Harker, Natural Historij of Igneous Rocks. 



