212 J. A. Thomnon — Rocks of Western Atisfralia. 



Basic Bi/kes. — Dykes of basic rocks are of frequent occurrence, not 

 only in the rocks of the auriferous series, but also in the gneisses, the 

 granites, and the older sedimentary rocks. Some of them penetrate 

 various members of the Nullagine Series, and maj^ be the feeders of 

 the volcanoes of that period. The only later period of vulcanicity 

 known in the State belongs to the Tertiary, but the rocks are confined 

 to the Kimberley District, and very little is known about them. It 

 is possible that the rocks under discussion belong to more than one 

 period. All that can be said of most of them is that they are later 

 than the folding or foliation of the granites and the deposition of the 

 gold, and anterior to certain faults. 



IV. IIesults of the Writer's Petrographical Studies. 



The writer's acquaintance with the rocks of Western Australia 

 commenced in London in 1908 with the preparation of a petro- 

 graphical report to accompany Bulletin 33 of the Geological Survey. 

 Some seventy rocks were examined and described in detail, but the 

 value of the report was lessened by the writer's ignorance of the 

 general features of Western Australian geology, and of the particular 

 occurrence of the rocks in hand. Subsequently a year was spent 

 (1909-10) as assistant to Dr. J. M. Maclaren in a private geologiciJ 

 survey of Kalgoorlie, the results of which are not yet published. 

 Some hundreds of slides of rocks from Kalgoorlie, Coolgardie, 

 Kanaona, Leonora, and Day Dawn were carefully examined. Before 

 leaving the State, the writer took the opportunity to look through 

 the large series of rocks collected by the Geological Survey, and the 

 Director, Mr. A. Gibb Maitland, kindly placed at his disposal 

 duplicate specimens of some two hundred selected rocks, many of 

 which had been analysed. Visits to Southern Cross and Albany 

 made up the writer's collection to over three hundred rocks. Sections 

 of these have been prepared and briefly examined with a view to the 

 preparation of this paper. Finally, in Sydney the writer was able to 

 examine, through the kindness of Professor David, a series of sixteen 

 rocks collected by the David Lindsay Expedition to Central Australia. 

 A paper embodying the results of this examination has been submitted 

 to the Royal Society of New South Wales. 



The majority of the rocks examined belong to the auriferous series. 

 A large number of them are coarse-grained massive amphibolites 

 which bear abundant internal evidence of their derivation from 

 basic intrusive rocks such as pyroxenites, hornblende dolerites, 

 olivine dolerites, dolerites, and quartz dolerites. The same rock 

 species are also represented by massive chloritic greenstones and by 

 fissile chlorite schists. Ultrabasic intrusions are represented by 

 sei'pentines, talc-carbonate schists, and magnesite rocks. All these 

 rocks appear to be intrusive into fine-grained amphibolites and 

 chloritic greenstones that contain little internal evidence of their 

 original structure, but must be regarded as forming a volcanic series. 

 A distinction may thus be made between older and younger 

 ampliibolites and greenstones. Within the contact aureoles of the 

 granites these distinctions are not so easy, for all the rocks have been 

 transformed into hornblende and talc-tremolite schists retaining no 



