J. A. Thomson— Rocks of Western Australia. 215 



of the basic dj^kes are only known to be post-granitic, some to be 

 post-gold, while a few are post-Nullagine. A comparison of their 

 petrographical character and state of alteration may show whether 

 more than one series is present, while their direction of alignment may 

 also help to diffei'entiate two or more series. Only one granite is 

 definitely known to be post-Mosquito Creek, but as this sedimentary 

 series contains gold reefs, it is probable that all the granites that can 

 be distinctly correlated with the deposition of gold belong here. 

 Many of the granites are rather inter-fuliation than post-foliation, for 

 they are gneissose along their margins, and the accompanying dykes 

 are considerably sheared. 



VII. Detailed Petrogkaphical Studies. 



The writer proposes to give the results of his detailed studies on 

 the rocks of the auriferous series, the jaspers, the granites, and the 

 basic dykes in subsequent papers. As a contribution to the knowledge 

 of the fundamental gneisses the following is offered. Little is known 

 of the geological occurrence of these rocks, but their structures favour 

 the view that they are highly metamorphic. 



Gr.S.M. 832. (This and subsequent numbers refer to the Register 

 of the Geological Survey Museum, Perth.) This is a well-foliated 

 biotite-microcline gneiss from Loc. 1046, N.W. of Mt. Dick, 

 Mortlock liiver, Noi'thara, i.e. in the western granite belt of 

 Woodward. The darker layers owe their colour to small plates of 

 biotite aggregated more or less in bands, and lying with their 

 cleavage planes in the planes of foliation. In the lighter- coloured 

 layei's quartz and felspars predominate. A plagioclase (oligoclase- 

 andesine) occurs sparingly in large elongated and much striated 

 crystals with its longer axes in the planes of foliation. Quartz also 

 occurs in elongated and similarly oriented crystals, separated by 

 numerous small rectangular plates of mici'ocline. An interesting 

 feature in the slide studied is a graphic intergrowth of two adjacent 

 crystals of quartz. Iron-ores are practically absent and accessories 

 rare. The rock has the composition of a microcline granite and the 

 structure of a gneiss. 



Gr.S.M. 2286^ is a hornblende-biotite gneiss from Dalgety Creek, 

 Lyons District, east of Gascoyne. It is not so perfectly foliated as 

 the preceding rock, for the biotite plates follow the boundaries of 

 large rounded slightly sericitic oligoclase crystals. Both the felspar 

 and tlie biotite exliibit bent cleavage-planes and twin-planes and 

 undulose extinctions. The biotite is frequently crowded with rutile, 

 sometimes in small stout ])risms, at other times in hair-like sagenitic 

 needles. Quartz is fairly abundant in small clear anhedral plates 

 between the felspars. There are no minerals with refringence less 

 than that of balsam, so that potash felspars are absent. A green 

 hornblende, subordinate in amount to the biotite, occurs in small 

 euhedral prisms. Apatite is the only accessory. 



G.S.M. 8692, from Goddard's Creek, ^ in the south-eastern corner of 



^ Some confusion exists as to whether the section described really belongs to 

 2286, owing to a mistake in labelling by the lapidary. 

 2 Cf. Bull, xxxvii, pp. 14-15, 1909. 



