BY H. C. RICHARDS. 191 



It is very difficult to fit in tlie above evidence with the 

 idea that the alkaline magma results from the assimilation 

 of limestone material as the Palaeozoic and Mesozoic for- 

 mations are intruded. It is conceivable that, in the magma 

 reservoir, the absorption of limestone material would bring 

 about a magmatic splitting resulting in an alkaline partial 

 magma being formed, but it has already been shown that 

 the sub-alkaline as well as the alkaline rocks are really 

 slightly deficient in lime and magnesia. 



It is of interest also to note that in connection with 

 Dal.y's theory Professor P. Marshall states**: — "There 

 appears to be no evidence in support of this theory as far 

 as our knowledge of the alkaline rocks of the South Pacific 

 Islands allows us to form a judgment at the present time." 



The evidence from this area is certainly strongly in 

 favour of differentiation of the original magma resulting 

 in the formation of the alkaline material having taken 

 place in the magma reservoir. The average composition 

 of the alkaline rocks in the area has been estimated, and 

 the Glass House ]\Iountains area, Mount Flinders area, and 

 the Main Range area were assumed to be of approximately 

 equal importance in making the calculation. 



The result is possibly a little high in soda and titania, 

 as the Main Range trachyte was especially rich in these 

 two, and perhaps more so than usual. Analyses used were 

 those of rocks numbered 2, 7, 9, 10, 12, 14, 16, 21. 



SuJ)-alkali)i€ Series. 



This series of rocks is represented in the upper, middle, 

 and lower divisions, and embraces something more than 90 

 per cent, of the extruded material. The series ranges from 

 acid to basic, and the rocks occur in all parts of the area. 

 In comparison with normal sub-alkaline rocks, it has been 

 shown that this series is characterised by a slightly higher 

 alkali percentage and slightly lower lime and magnesia 

 percentage ; also, while the alumina is lower and the iron- 

 oxides higher, the combination of these is about normal. 



** Trans, and Proc, N. Z. Inst., vol. xlvii., 1915, p. 372. 



