Vol. 69.] PETROLOGr OF THE KALGOORLIE GOLDFIELD. 663 



tout, while the ophites show a uralitic change, the lherzolites are 

 frequently serpentinized. The hornblende-schists and serpentines 

 •of the Lizard form an analogous case. On grounds of state of 

 alteration, consequently, there is no reason to separate the ser- 

 pentines from the quartz-dolerite series as ' newer eruptives.' 



These remarks apply with still greater force to the derivatives of 

 .peridotites occurring in the ' North End.' These consist partly 

 ■of talc-carbonate schists in the formation of which considerable 

 pressure must have acted, and partly of magnesite-rocks and 

 fuchsite-magnesite-rocks as the metasomatic products in the areas 

 traversed by auriferous veins. They are, therefore, pre- foliation and 

 pre-gold rocks, in the same sense as the rocks of the quartz-dolerite 

 series. 1 



Further, the distribution of the peridotite and pyroxenite 

 derivatives in the dumps of the eastern part of the ' North End ' is 

 such as to suggest that the pyroxenites occur as narrow bands in 

 •a large mass of peridotite : a mode of occurrence compatible with 

 the later intrusion of the pyroxenite, or with the contemporaneous 

 intrusion of the two rocks as a banded complex, but scarcely with 

 'the later intrusion of the peridotite. 



It is most probable, therefore, that the peridotites form the 

 ultrabasic pole of the quartz-dolerite series, in support of which 

 view many analogies might be quoted. The only observation 

 that creates any difficulties is that the peridotite-intrusion in the 

 -eastern part of the ' North End ' has determined a con tact- alteration 

 in the surrounding fine-grained greenstones, while the larger 

 •quartz-dolerite intrusion has apparently no such aureole. This 

 fact, notwithstanding, the bulk of evidence suggests that the 

 peridotites were , the earliest intrusives, followed closely by the 

 pyroxenites, and at an interval by the quartz-dolerite and its local 

 •variants. 



(b) Relationship of the Porphyries and Porphyrites to the 

 Quartz-Dolerite Series. 



The albite-porphyries form narrow dykes running north-north- 

 west and south-south-east in the quartz-dolerites, which have the 

 same general trend. Both rocks are locally schistose, and the 

 planes of foliation strike parallel with the trend of the rocks. It 

 is, therefore, obvious that the shape of the intrusions of both types 

 -of rock was determined by pressures similarly directed to those 

 which subsequently brought about their schistosity. Both must 

 have been intruded during intervals of relief of pressure in the 

 •course of one great epoch of earth-pressure. On the whole, this 

 mode of occurrence is in favour of a close age-relationship, and 

 therefore of the probability of a magmatic relationship between 

 the two rocks. Nevertheless, in contact-altered goldfields of the 

 Coolgardie type a somewhat similar mode of occurrence is observed. 



1 See J. A. Thomson, ' The Classification of the Rocks of the Western 

 Australian Goldfields ' Geol. Mag. dec. 5, vol. ix (1912; pp. 213-14. 



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