662 DK. J. A. THOMSON ON THE ["Dec. I9I3, 



Whether the jasperoid rocks have arisen hy a surfaee-silicification, 

 as their absence in depth might suggest, or whether they are 

 lenticular bodies following one another en echelon within the 

 bands of graphitic schists, and thus not necessarily present in all 

 cross-sections of the bands, cannot be decided on the evidence 

 available. Dr. Lindgren ' has suggested a deep-seated origin for 

 the hsematite-jaspers found in other goldfields of the State. 



IV. Relationship op the Rocks. 



While it is a matter for argument whether or not all the intrusive 

 rocks from Kalgoorlie described above are related by differentiation 

 from a common magma, they may be divided into groups, each of 

 which consists only of members showing undoubted affinity one 

 with the other. 



Group a consists of tbe quartz-dolerites, dolerites, hornblende- 

 dolerites, and pyroxenites. The first three of these rocks form 

 diffeient parts of the same mass, and show gradations from one to 

 the other. The pyroxenites, although found in separate intrusions, 

 betray their relationship with the quartz-dolerite by the occa- 

 sional presence of micrope^niatite. This group will be termed 

 in the subsequent discussion 'the quartz-dolerite series.' 



Group /3 contains the porphyrites and albite - porphyries. 

 These rocks form a well-defined series with almost every gradation 

 between the extremes represented, and their close relationship 

 cannot be doubted. 



Group 7 contains by exclusion the peridotites. 



Mr. Simpson 2 separated the last two groups from the first as 

 'newer eruptives,' his reasons being presumably that the albite- 

 porphyries traverse the rocks of the quartz-dolerite series in 

 well marked dykes, and that the serpentines of Hannan's Lake 

 are relatively little-altered igneous rocks as compared with the 

 amphibolites. 



a) Relationship of the Peridotites to the 

 Quartz Dnlente Series. 



The presence of the peridotites was first established by Mr. 

 Simpson after a study of the serpentine and carbonated serpentine 

 of Hannan's Lake. The interpretation ol serpentines as altered 

 peridotites is so well known, that one naturally regards a serpentine 

 as an intrusive rock. The interpretation of amphibolites, on the 

 contrary, has lagged behind ; and, in consequence, tlmse who have 

 not made a study of the subject would tend to regard serpentine 

 occurring among amphibolites as an intrusion into them. This is 

 far from being necessarily the case. For instance, the lherzolites 

 and ophites 01 the Pyrenees helong to the same magmatic series ; 



1 'Econ. Gieol.' vol. i (19<>6j p. 542. 



2 Bull. Geol. Surv. W. Austral. No. fi (1902) p. 72. 



