126 PETROLOGY OF ALKALINE ROCKS 



them to the kenytes and other alkaline rocks in which phosphoric acid is, as a rule, very 

 deficient. 



The Cape Bird rocks indicate that at this reservoir of magma there was an early 

 differentiation into a hornblende basalt magma and an alkaline kenytic magma, each 

 of which has again further differentiated and the final products obtained were mixtures 

 of the differentiates of these two magmas. 



Original magma. 



A, Hornblende basalt. B, Kenyte. 



Aj, Limburgite. A 2 , Olivine basalt. A 3 ,Kulaite. B a ,Enstatite basalt. B 2 , Kenyte. B 3 , Trachydolerite. B 4 , Trachyte. 



Basanite and Hornblende 

 Tephrite. Trachyte. 



(A 2 + B 3 ) (A 3 + B 4 ) 



The rocks of Cape Bird suggest that an examination of Mount Bird will reveal a 

 much more complete series of acid alkaline eruptives than has been poured from the 

 Erebus vent. 



The abundance of basalt hornblende and pseudomorphs after this mineral in the 

 Cape Bird rocks, and their comparative absence in those of the Erebus series (except 

 at Cape Barne), in spite of the similarity in other respects of both series of rocks, is a 

 fact of some significance. It may, perhaps, be an indication that differentiation of the 

 Cape Bird magma commenced in much more deep-seated regions of the earth than 

 the Erebus magma, and the mixing of differentiation products, as already suggested, 

 might have taken place in the process of eruption. 



The basaltic hornblende occurs, according to Prior, in certain basalts of Cape Bird 

 and in certain trachytes from Hut Point and Observation Hill. I also found pseudo- 

 morphs after hornblende in some Cape Barne basalts. 



As may be gathered from the foregoing notes the nature of the pseudomorphs varies 

 considerably. In some cases they consist wholly of magnetite, in others principally 

 of hypersthene ; in some of a, mixture of grains of magnetite, hypersthene, and colourless 

 augite ; in some nuclei of the original basalt hornblende occur ; in some a secondary 

 deep red to opaque pleochroic hornblende occurs with magnetite, augite, and felspar ; 

 this secondary hornblende has been referred to by Prior as a cossyrite-like hornblende, 

 but in my slides it has typically the characteristics of kaersuetite, though in some cases 

 it is very like cossyrite. The most extreme stage of alteration of hornblende seems to 

 be into a mosaic of magnetite augite and felspar, and where this occurs chemical inter- 

 action between the constituents of the hornblende and of the matrix has probably 

 taken place. 



Hornblende, more or less pseudomorphed, has also been described by Prior in rocks 

 from Mount Terror, so that this mineral seems to be widely distributed in all the rocks 

 except those of Erebus. That the volcanic rocks collected by Borchgrevink and by the 

 Scott expedition at Cape Adare are closely analogous to those of Ross Island is a fact 

 which is of great interest, and is further emphasised by the important collection of 

 erratics described by Woolnough. 



