THE QUARTZ DOLERITES 255 



by snow by high-level air currents they would be starved of snow supplies. It is 

 probable, for reasons already stated, that even when the belt of glaciation had 

 spread equatorwards to about the parallel of 40° S., as was probably the case in 

 Permo-Carboniferous time, there might still then, as now, be supplies of snow carried 

 by high-level cyclonic circulation to the South Pole. Tundra conditions would be 

 most unfavourable to the development of coal, and one must assume that at the 

 time the coalfields of the Beardmore Glacier were being deposited the climate, though 

 possibly cold, was humid. Rain of course now never falls within this region. There 

 must necessarily have been a rainfall, and therefore a more genial climate than at 

 present, in order to support the growth of coal-forming vegetation, including pei'haps 

 coniferous trees, the existence of which the fossils from the Beardmore Glacier may 

 suggest. If, then, the Beacon Sandstone and its coal-measures are to be classed 

 provisionally as Gondwana, they probably belong either to the top of the Permo- 

 Carboniferous formation, such as the Newcastle Coal-measures, or to the coal- 

 measures of the Trias-Jura system. 



QUARTZ DOLERITE 



The quartz dolerites which form such conspicuous members of the cliff sections of 

 the great horst have been described in detail with analyses at the end of this Memoir 

 by Mr. W. N. Benson, B.Sc. They occur in the form of huge sills, like that of Finger 

 Mountain in the Ferrar Glacier Valley, cutting obliquely across the planes of bed- 

 ding of the Beacon Sandstone. 



The quartz dolerite is characterised by interstitial patches of granophyre ; in 

 places it contains rhombic pyroxene, and in its amount of potash closely approaches 

 to essexite. The predominant pyroxene in some varieties is hypersthene, other types 

 pass over into olivine and norites. In the typical quartz dolerites both rhombic 

 and monoclimic pyroxenes are present, the rhombic forms being represented by 

 bronzite or enstatite. From the presence of micropegmatite and nearly uniaxial 

 enstatite augites these rocks show great resemblance to the Konga type of diabase 

 so well known in Sweden. The presence of micropegmatite has already been re- 

 corded by G. T. Prior.* Mr. Benson speculates as to whether the gi'anophyric 

 inter-growths are derived from intrusions of gi'anophyre into normal gabbros or 

 dolerite, or are formed by magmatic mixing, or again are due to the intrusion of 

 the acid residuum from the crystallising basis rock. Mr. Benson inclines to the 

 thii-d view. He discusses the parallel distribution of quartz dolerite in Tasmania, 

 South Africa, and parts of South America. He concludes that " the similarity be- 

 tween 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." 



Dr. G. T. Prior and Professor W. G. Woolnough have already pointed out the 

 * Geological Memoir of the National Antarctic Expedition, 1901-4, p. 136. 



