OF MOUNT EREBUS, ANTARCTICA 119 



In some cases magnetite is the sole alteration product recognisable in the pseudomorphs. 

 In other cases several of the above-mentioned minerals are present. 



The formation of red secondary hypersthene by the molecular change of hornblende 

 has been ably described by Washington * in several papers dealing with the rocks of 

 Kula, in Syria. 



The Cape Bird rocks consist of: 



(1) Trachytes of a strongly alkaline facies, which generally contain pseudomorphs 

 after basaltic hornblende in greater or smaller amount. 



(2) Kulaites or trachydoleritic rocks with the peculiar pseudomorphs after basaltic 

 hornblende which characterise kulaite. 



(3) Subalkaline basalts and dolerites, some enstatite- bearing, some olivine-bearing. 

 The rocks of the kenyte series are absent in the Cape Bird rocks which have come 



under my notice. It is, however, very probable that the complete Erebus series will 

 recur on Mount Bird, with this difference alone, that basalt hornblende and its 

 pseudomorphs will be present in all the rock species in varying proportions. 



In most of the Cape Bird rocks examined the basalt hornblende has been altered 

 by the magma in such a manner that there can be little doubt that it is not a normal 

 differentiation product of the rocks in which it occurs. The subalkaline magma has 

 differentiated in the first place into an alkaline magma and a basic one, in the latter of 

 which hornblende was a normal consolidation product. The basic magma has then been 

 injected into the alkaline, and has been mixed with it in varying proportions whilst 

 differentiation of the alkaline magma was in progress, the hornblende undergoing partial 

 resorption and alteration. Then the differentiated alkaline lavas were extruded and 

 rapid cooling caused the final metamorphosis of the hornblendes. 



P. 314 (622). Hand-specimen missing. Alkaline Trachyte, Cape Bird. — This is 

 a beautiful macrocrystalline trachyte showing under the microscope a well-marked 

 flow structure. 



Texture practically holocrystalline, dopatic, porphyritic serial in small felspar 

 phenocrysts, some of which are visible to the naked eye. There are also stray pheno- 

 crysts of corroded magnetite pseudomorphs after rod-shaped hornblende crystals. 

 These are minophyiic. 



The porphyritic felspar (magniphyric) consists of anorthoclase, as shown by the 

 fine multiple twinning and moiree accompanying a sanidine habit. The fine acicular 

 felspars of the base are probably also anorthoclase. The other minerals consist of 

 corroded grains of magnetite and a^girine-augite. A little isotroj)ic felspathoid occurs 

 interstitially. 



The rock, but for the hornblende pseudomorphs, is identical with some Mount Cis 

 trachytes. 



P. 341 (653). This is a fluidal fine-grained hemicrystalline trachyphonolite 

 containing, in addition to the minerals of P. 314,nepheline and corroded olivine grains, 

 as well as a number of grains of camptonitic minerals appertaining to the inclusion 

 described in this rock by Mr. J. Allan Thomson. 



J. 18 (1205). Altered Phyro -hornblende Trachyte, Cape Bird. (For Analyses see 

 Table IV, A, p. 120 ; Table V, No. VI, p. 122 ; and Table VI, No. VI, p. 123 ; 

 Photo, Plate III, fig. 5). Megascopically this is a light grey rock dotted with black 

 rod-shaped phenocrysts. The structure is porphyritic hiatal, and slightly vesicular. 



* Jour. ofGeol., vol. viii, 1900 ; Am. Jour. ofSci., vol. xlvii, 1894 : The Volcanoes of the Kula 

 Basin in Lydia, 



