98 



PETROLOGY OF THE ALKALINE ROCKS 



that eegerite commenced to separate. Felspar then finished crystallising, after that 

 segirite, which partly is later than felspar. Lastly the felspathoids formed, and occur 

 mainly in interstices as sodalite and analcite. 



Chemical Composition. — Table I gives the analyses of pumice * from the top of 

 Mount Erebus and of the trachyphonolite J. 13. For comparison the analyses 

 are recorded of phonolitic trachyte, Cape Adare ; phonolitic trachyte, Mount Terror 



TABLE I.— ANALYSES OF TRACHYTIC ROCKS 



(Cape Crozier) ; hornblende trachyte, Observation Hill ; and traehyandesite from the 

 Warrumbungle Mountains, New South Wales. 



In Table VI (p. 123) the magmatic names of the rocks are given. 



It will be seen that the pumice from the summit of Erebus is closely allied in 

 composition to the hornblende trachyte of Observation Hill, described by Prior, f 

 Our specimen, J. 3 (1913), is almost identical with Prior's, so that his analysis probably 

 represents the composition of J. 3. These rocks are closely allied on the one hand to 

 the tephritic trachytes, as that of Farodada, Columbretes, described by Becke J and 



* The pumice analysed is the ground-mass of a very beautiful kenyte which forms the latest 

 lava extravasation of the present active crater of Mount Erebus. Some specimens contain abundant, 

 large, handsome, twinned crystals of anorthoclase. — D. M., Ed., 1916. 



t National Antarctic Expedition, 1901-4, " Petrology." 



% Tschermak's Min. und Petr. Mitth., 1906-7, Bd. xvi, p. 174. 



