214 J. A. Thomson — Rock& of Western Amtralid. 



or after the period of foliation of the rocks into which it is intrusive. 

 To attach an age significance to this relationship, it is necessary to 

 assume that the foliation is of the same general age over the whole 

 State. This is a perfectly justifiable assumption in the present state 

 of our knowledge. Traces of a double foliation have been observed in 

 gneiss at Big iSherlock River, Pilbara Goldfields, by Cadell : ' "The 

 older and coarser banding has a north and south strike at Big 

 Sherlock Station, and at right angles to this there are zones of 

 secondary and much finer foliation, within whicli it has the character 

 of a niylonite," etc. With this exception there is no evidence as yet 

 brought forward to show two distinct periods and directions of 

 foliation such as exist in the Lewisian and Dalradian of Scotland. 

 Variations in strike between different districts amount to nearly 90°, 

 but whether this is due to different systems or to curving trend-lines 

 is uncertain. A map of the State showing the trend-lines would 

 be a great boon. For the present we may regard the foliation as of 

 one age. 



5. If it can be shown that its date of intrusion is anterior or . 

 posterior to the formation of auriferous quartz-reefs. Several distinct 

 modes of gold occurrence exist in the State, and these may not 

 necessarily be of the same age. jS"evertheless. the presumption is, as 

 suggested by W. Lindgren,* that the mise -en-place of the gold closely 

 followed the intrusion of the granites, and belongs to one period. 



6. If it can be shown that its intrusion is anterior or posterior 

 to faults of known age. Little is yet known as to the age of the 

 various systems of faults, but this line of evidence may yet yield 

 important results. 



VI. Suggested Classifications. 

 From the application of the above-mentioned considerations, the 

 classification outlined below is suggested. It is put forward as 

 a working hypothesis and will serve a useful purpose if it directs 

 attention to the gaps in our knowledge. The rocks are arranged 

 in ascending order of age. 



Fundamental gneisses and 

 schists. 

 fa. Warrawoona sedimentary series. 

 „ „ ,. ^. i ^'\h. Older amphibolites (volcanic). 

 rA. Pre-fohation -, j-«. Later amphibolites (intrusive). 

 ■ \b. Older granites and porphyries. 



4. Jaspers. 



5. Mosquito Creek Series. 

 i 6. Newer granites and porphyries. 



^B. Post-foliation \ 7. Gold deposits. 



\ 8. Post-gold acid intrusions. 



( (a. Nullagine sediments. 



II. Nullagine Series - 9.- Z>. Nullagine volcanics and their 



[ [ feeding pipes. 



III. Post-Nullagine 10. Basic dykes. 



The principal gaps in our knowledge relate to the granites, 

 porphyries, and basic dykes. These may be of many ages. Some 



' Trans. Edin. Geol. Soc, vol. vii, p. 181. 

 - Econ. Geol., vol. i, p. 530, 1906. 



I. Pre-Nullagine 



