PART IX 



REPORT ON THE PETROLOGY OF THE 



DOLERITES 



COLLECTED BY THE BRITISH ANTARCTIC 

 EXPEDITION, 1907-1909 



(With Plate) 

 BY 



W. N. BENSON, B.Sc. 



The dolerites of South Victoria Land extend over a wide area. They are developed 

 in the Royal Society and Prince Albert Ranges, on the western margin of the Great 

 Ross Barrier and the Ross Sea. They have been proved in the Mount Nansen region 

 and in the Ferrar Glacier valley to extend to more than sixty miles from the coast. 

 An examination of the inclusions in the lavas of Mount Erebus show that they underlie 

 Ross Island, and moraines on the flanks of the mountain itself contain many dolerite 

 erratics, probably brought from far to the south. They also occur in the moraines 

 on the mainland. Pebbles of dolerite similar to these here described have been dredged 

 up off Cape Wadworth, and to the south of the Balleny Islands. The dolerites occur 

 in dykes, necks, and sills in the basement complex, and form huge sills up to two thousand 

 feet in thickness in the overlying Beacon Sandstones. They have been met with to a 

 height of seven thousand feet above sea-level. 



The collections of the expedition of 1901-1904 were examined by Dr. G. T. Prior, 

 and the following notes may be considered as supplementing his descriptions.* The 

 material studied by the present writer was collected chiefly from the moraines at Cape 

 Royds, on Ross Island, but a few specimens were submitted from the Stranded Moraines, 

 the Ferrar Glacier valley, and at points visited by the northern party. Several interesting 

 features have been noted that were not mentioned in the previous report. Chief among 

 these is the abundance in the quartz-dolerites of the peculiar pyroxene enstatite-augite, 

 the occurrence of rhombic pyroxene in granular and porphyritic varieties, and in the 

 presence of a passage rock into the essexites. 



A convenient subdivision of the rocks studied is into the phaneric and aphanitic 

 groups. The former have a medium grain-size, with an ophitic or gabbroid structure ; 

 certain of them are distinctly porphyritic, through the development of phenocrysts of 

 pyroxene or plagioclase. The aphanitic rocks are very finely grained and have a granu- 

 litic structure, occasionally becoming slightly ophitic. 



The rocks are composed essentially of basic plagioclase and pyroxene, chiefly mono- 

 clinic, with a varying amount of a micropegmatitic intergrowth of quartz and felspar 

 occurring interstitially. They thus correspond to the Hunne-diabas of Tornebohm, but, 



* To be in the forthcoming Geological Memoir of the Expedition, The National Antarctic 

 Expedition, J 901-4, vol. i, pt. ii, chap. v. 



n 153 Y 



