REPORT ON THE PETROLOGY OF THE DOLERITES 159 



fine grained, felsitic, and in patches so distinct from the rest of the rocks as to suggest 

 a mingling of two rock types. 



Another point of interest is the widespread distribution of quartz-dolerites, occurring 

 under conditions analogous to those found in the Antarctic. In Tasmania, South Africa, 

 and in parts of South America quartz-dolerites are found forming sills in sandstone 

 and other formations which have intruded during the Mesozoic period. 



In particular the similarity between the occurrence of dolerite in Tasmania and 

 South Victoria Land is so striking as to strongly support the view that the two areas 

 form part of a geological unit. Tasmania is chiefly composed of an almost horizontally 

 bedded series of sandstones, coal-measures, mudstones, and limestones of Mesozoic and 

 Permo-carboniferous age, lying on a strongly folded complex of Silurian and older 

 rocks intruded by granite. In Cretaceous times came the intrusions of dolerite,* which 

 formed great sills in the Permo-carboniferous and particularly in the Mesozoic sand- 

 stones. After the intrusion the area was broken considerably and differentially elevated 

 with block-faulting. Erosion has now exposed the dolerite which occurs, capping nearly 

 every important elevation in the island. The geology of the Tasmanian and Antarctic 

 dolerite is thus very similar, and the physiography of the two areas is remarkably constant. 

 Microscopically there is also a strong resemblance between the rocks. Dr.Woolnough first 

 drew my attention to the similarity of the Launceston rock to some of the Antarctic dole- 

 rites, and the examination of a series of Tasmanian dolerites, kindly sent by Mr. W. H. 

 Twelvetrees, the Government Geologist of Tasmania, fully bears this out. Enstatite-augite 

 has been noted in rocks from several other localities than Launceston, in which it was 

 discovered by Professor Osann, and may be taken to be of fairly general occurrence. 

 The mesostasis is usually of the fine-grained felsitic type, but coarse-grained micropeg- 

 matite was noted in a rock from North-West Bay by Dr. F. P. Paul f (see Analysis V). 



The biotitic variety of dolerite from the Antarctic may have its parallel in the biotitic 

 rock from Dundas described by L. K. Ward.f 



A further point of similarity is in the general absence of olivine from the two areas. 

 The analyses given (Nos. IV and V) are the only ones of Tasmanian dolerites that are 

 available ; they would indicate that the percentage of magnesia was less than in those 

 of the Antarctic. According to Mr. W. H. Twelvetrees, in Tasmania also there is an 

 absence of veins of granophyre from the dolerite masses. 



In South Africa the dolerites were intruded in Jurassic times into the horizontal or 

 slightly folded beds of the Karoo system, in which they form huge sills up to two thousand 

 feet in thickness. ' The constituents are plagioclase, felspar, augite, olivine, and 

 magnetite in the relative order of their abundance ; but olivine is not infrequently 

 absent. In addition, biotite may be present and sometimes original hornblende, either 

 independently or in close connection with the augite. In the more acid types quartz 

 and occasionally orthoclase felspar are present, often in the form of micropegmatite." § 

 In certain of these dolerites Wahl has noted the presence of enstatite-augite. The 

 chemical composition of the dolerites is very uniform, but the analyses given by E. Cohen 

 are invariably much higher in ferric oxide and lower alumina than those of Antarctic 



* It should be remarked that the Tasmanian geologists follow the German usage in terming 

 these rocks " Diabase." For a general description of their mode of occurrence and petrology 

 see W. H. Twelvetrees and W. F. Pettard, Proceedings of the Royal Society of Tasmania, 1898-99, 

 p. 47, and W. H. Twelvetrees, " The Igneous Rocks of Tasmania " Report of the Australian Asso- 

 ciation for the Advancement of Science, 1902, p. 287. 



| " Beitrage zur-petrographischen Kenntniss einiger foyaitischentheralitischen Gesteine aus 

 Tasmanien," Tschermak's Min. und Petr. Mitt., Bd. xxv, p. 267. 



J Tasmanian Geological Survey, Bulletin No. 6, p. 28. 



§ A. W. Sogers and A. L. Du Toit, The Geology of Cape Colony, chap. ix. 



