260 STRATIGRAPHY 



orthoclase and nepheline (pseudo-leucite), and vary in size from just megascopic to 

 microscopic (minophyric to miniphyric)." Di-. Jensen concludes that " one of the most 

 striking points about the Antarctic lavas examined is the highly differentiated nature 

 of the effusive products. The abundance of limburgites, magnetite basalts,. olivine 

 nodules, sanidlnite inclusions, and the occurrence of basalts without olivine, felspar 

 basalts, and olivine basalts, in different interbedded flows, show that differentiation 

 was very complete, and that the eruption tapped now one and now another portion 

 of the magma." He points out that "the same highly differentiated nature of 

 Antarctic eruptive rocks is evidenced by the alkaline, sub-alkaline, and basic lavas 

 obtained by Borchgrevink at Cape Adare," and that " there seems to be little 

 difference, if any, between the alkaline volcanic rocks of Ross Island and those of 

 the mainland. He adds that " the sequence of these lavas appears to have been the 

 same as that which is so characteristic of the Australian alkaline province, that is, 

 from acid to basic." 



The rocks collected at Cape Bird by Dr. Mawson were not obtained in situ, but 

 had evidently not travelled far from that locality. They are described by Dr. 

 Jensen as strongly alkaline trachytes, generally containing pseudomorphs after 

 basaltic hornblende, kulaites or trachydolerite rocks with the opaque pseudomorphs 

 after basaltic hornblende which characterise kulaites, sub-alkaline basalts and dolerites, 

 some enstatite-bearing, some olivine -bearing. 



Dr. Jensen suggests that the rocks of Cape Bird, on further examination, will 

 probably reveal " a much more complete series of acid alkaline eruptives than has 

 been poured from the Erebus vents." The important collection of erratics described 

 by Dr. Woolnough shows that alkaline granitic rocks are represented on the main- 

 land of South Victoria Land. As these granites are mostly, if not wholly, older than 

 the Beacon Sandstone, the age of which latter is provisionally considered to be Pre- 

 Gondwana, the region of South Victoria Land and Ross Island must have been 

 marked out early in geological time as an important province for alkaline rocks. 



We would invite special attention to Dr. Jensen's conclusions as to the sequence 

 of effusive products in Ross Island at different levels. He emphasizes the important 

 part played by specific gravity, " the molten magma has differentiated in the magma 

 reservoir, and probably tongues of differentiated rocks have been thrust into sur- 

 rounding formations. Fissures may tap one or all of the differentiation products, 

 but the more elevated vents situated over the centre of the magma chamber will 

 tap the lavas in the order of their specific gravity, and will continue to pour 

 out each type of lava much longer than the subsidiary vents and fissures. Thus, if 

 Cape Barne represents lavas poured from the fissure, which has tapped sill-like off- 

 shoots of the main reservoir, the whole sequence might be completed in this locality 

 while the summit of Erebus is still in the kenyte phase of eruption and the summit may 

 become extinct before emerging from this stage, the forces being insufficient in the 

 final stages of eruption to raise the subjacent basic lavas to the summit, so that these 



