PART VII 



REPORT ON THE 



PETROLOGY OF THE ALKALINE ROCKS OF 



MOUNT EREBUS, ANTARCTICA 



[With Five Plates) 

 BY 



H. I. JENSEN, D.Sc. 



Through the kindness and courtesy of Professor David, F.R.S., C.M.G., Sir Ernest 

 Shackleton, and Mr. R. E. Priestley, F.G.S., of the South Pole Expedition of 1907-9, 

 I have been able to examine and classify the alkaline eruptive rocks of Mount Erebus 

 and Ross Island. In this work I have been greatly assisted by Mr. R. E. Priestley. 

 He cut sections and made a preliminary examination of a much larger number of 

 specimens of these rocks than I had time to devote attention to, and all the most 

 important types which he discovered he transferred to me for further investigation and 

 classification. 



The eruptive rocks of Ross Island have microscopically, with very few exceptions, 

 strongly alkaline affinities. The exceptions consist entirely of olivine basalt, which, 

 though not an alkaline rock and commonly associated with calcic magmas, may yet 

 be a differentiation product of an alkaline magma. The olivine basalts do, I understand, 

 burst through the kenytes and other alkaline types, and are, where found, the final 

 products of volcanic activity. The other basic rocks, poor in alkali, such as limburgite 

 and magnetite basalt, are nevertheless types which are so commonly associated with 

 alkaline magmas and so rarely with calcic magmas that they are generally looked upon 

 as normal differentiation products of the former. 



All the rocks dealt with in this part may be regarded as differentiates of a magma 

 of the composition of intermediate kenytes. The origin of the kenyte itself is a more 

 difficult problem. 



In my paper on " The Distribution, Origin ... of Alkaline Rocks," * I infer from 

 the sequence of Australian alkaline lavas that " basic lavas may rise along fractures 

 and intrude themselves into the alkaline zone" of the earth's crust (for the supposed 

 existence of which several reasons were put forward in the same paper). " Here they 

 will assimilate alkali, and also help to bring about a refusion of the whole alkaline 

 zone." In this paper therefore the origin of basic alkaline rocks is ascribed to stoping, 

 assimilation, and solution of alkaline sedimentary and metamorphic rocks by a basaltic 

 magma from the zone of basic igneous rock, which is supposed to underlie the whole 

 of the earth's crust. 



Reginald A. Daly, in a paper on " The Origin of Augite-Andesite . . ." f published 



* Proc. Linn. Soc. ofN.S.W., vol. xxxiii, pt. iii, 1908. 

 f Jour. ofGeol., July- August 1908. 

 II 93 P 



