36 



BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. 



[Memoirs National 

 I Vol. XIX, 



ultrabasic and basic rocks lie to the west of this (Twelvetrees and Ward '10, Hills '14, Water- 

 house '16). Generally the gabbro and peridotite (now serpentine) are so closely associated that 

 they are not separately mapped, but in the South Heemskirk region they form areas separate 

 from each other and from the granite to which they are roughly peripheral (Waterhouse '16). 

 However, there is generally a marked contrast between the approximately concordant or sill- 

 like masses of basic and ultrabasic rocks and the transgressive boundaries of the granite. Reid's 

 ('21) more detailed mapping of these areas reveals the presence of several laccolitic differentiated 

 complexes of peridotite, pyroxenite and gabbro, either quite separate from the granite or invaded 

 by it. That of the Heazelwood District (fig. 9) appears to dip generally toward the southeast, 

 and the succession of peridotite, pyroxenite, and gabbro in this direction recalls Bowen's explana- 

 tion of the origin of the Tagil complex in the Ural Mountains. Osmiridium and diamonds occur 

 in the Heazelwood peridotite. All these lie in the western districts, but sill-like masses of 

 peridotite-serpentine have also been found in the south of the island invading Cambrian quartzite 

 (Twelvetrees '09) and in the north between Cambrian quartzite and ancient amphibolite-schists 

 (ibid. '02), and again in a mass a mile in width and 4 miles in length, invading, apparently con- 

 cordantly, the lower Paleozoic sandstones, and in turn invaded by dikes of granite (ibid. '17). 



+ .+ 



Flo. 9.— Geological sketch-map of the Heazelwood District, northwest Tasmania. (After A. M. Eeid.) 



1. Invaded Ordovician lavas and Silurian sediments and limestone. 



2. Devonian peridotite. 



3. Pyroienite 



4. Bronzitite. 



5. Gabbro. 



6. Granite. 



Central and eastern Tasmania is occupied by an unbroken series of conformable Permo- 

 Carboniferous and Mesozoic sediments, which are nearly horizontal and according to Nye 

 ('20, '21) have been invaded in their lower portions by an immense sill-like mass of quartz- 

 dolerite from which rise minor intrusions of various forms including large and small dikes and 

 sills. These are comparable in many respects with the dolerites of Antarctica, the Karroo, and 

 the Newark series of New Jersey. (Twelvetrees and Petterd '99, Twelvetrees '02, Thomson '11, 

 Benson '16.) Their intrusion was clearly unaccompanied by any crust folding. 



Teale ('19) has described in central Victoria, near Mansfield, a series of Cambrian Proto- 

 spongia cherts. These contain normal and richly albitic diabase, the spilitic nature of which is 

 apparent. In addition, there are diabasic tuffs and agglomerates, together with small masses of 

 serpentine. The whole igneous complex has been so strongly silicified that the tuff has been 

 more or less changed to chert, while the diabase has in places become a red jasper. The linear 

 arrangement of the masses of jasper suggests the occurrence of a shear or fracturing zone which 

 favored at recurrent intervals the access of silicifying solutions. A similar complex occurs at 

 Heathcote (Skeats '08) and probably in several other parts of Victoria. 



