Chapter VI. 



THE OCCURRENCE OF RASIC AND ULTRARASIC INTRUSIVE ROCKS 



IN AUSTRALASIA. 



The summaries of the structure of Australasia given by David ('11) and Andrews ('16), 

 show that some analogy exists in its general plan with that of Asia, in that it has on one side an 

 ancient nucleus, the massif of Western Australia, against which the Paleozoic and later rocks 

 have accumulated and have been ridged into a series of roughly concentric folds of progressively 

 more recent date as we recede from the nucleus. 



The recent investigations of the Western Australian Geological Survey in the Coolgardie 

 region show that the most ancient rock is gneiss, upon which rests a mass of amygdaloidal 

 basic lavas, perhaps pillowy, together with porphyrites and rhyolites, associated with slates 

 and conglomerates. These together make up the ancient green stones, and after a period of 

 intense folding, they were invaded by the great series of plutonic rocks that now occupy the 

 greater part of the surface of the Western Australian plateau, consisting of peridotite and gabbro 

 in small amount, diorite, syenite and abundant granite, with dykes of pegmatite, porphyry, 

 and diabase (Honman '17). This may be taken as generally typical of other areas in the state 

 where an extensive sequence of events has been proved. The peridotites appear not only in 

 the region cited, but in most of the carefully studied areas that lie in a broad belt extending to 

 the northwest coast at Roebourne, and toward the south coast at Port Esperance, a distance of 

 nearly a thousand miles, over which the strike of the gneisses remains almost constant. They 

 have been described especially from the following districts: Pilbara (Maitland '08), Meeka- 

 tharra (Clark '16), Phillips River and Norseman (Woodward '09). Much younger and massive 

 intrusions of norite have been found in several areas, notably in the last mentioned. They also 

 occur in some variety in the Cavanagh Range near the boundary between Western and South 

 Australia. (Thomson '11.) A further example occurs near the Murray River in South Aus- 

 tralia, and was described by Chewings ('94). This last rock, like the other norites, is very 

 much less altered than are the rocks in the dikes of diabase, which occur in north and south- 

 central South Australia, and were, probably, erupted during an early Paleozoic period of orogeny 

 (Benson '09). Thomson, indeed suggested that the dolerites may be coeval with the late 

 Mesozoic dolerites of Tasmania, and that their eruption may have been connected with the 

 crust-movements during the breaking-up of Gondwanaland. 



No ultrabasic rocks are known in South Australia or the Northern Territory, but they 

 appear in each of the four eastern States. In Tasmania, great folding occurred in the pre- 

 Cambrian and late Ordovician times, but was not then accompanied by the intrusion of basic 

 rocks. Folding again took place between Silurian and Permo-Carboniferous (Permian) times 

 accompanied with abundant plutonic intrusions. In the absence of any Devonian and Car- 

 boniferous sediments, the period of folding has been held to be Lower Devonian, coeval with that 

 of Victoria, during which extensive granodioritic intrusions occurred (Skeats '09). 



The plutonic rocks of Tasmania range from ultrabasic to acid types, but intermediate 

 types are very rare. In general, the basic and ultrabasic types form long intrusions approxi- 

 mately parallel to the general strike of the invaded Silurian and older rocks. The largest masses 

 appear to extend in a belt about 30 miles from south to north and from less than 2 to 5 miles 

 from east to west, running intermittently from the Dundas to beyond the Waratah mining field 

 (Twelvetrees '14). It is invaded by granite, and in the southern portion is divided into two 

 parallel belts (Ward '09, '11, Waterhouse '14, Conder '18). Other less continuous masses of 



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