BY H. 1. JENSEN. 



575 



Before dismissing the question of the origin of the alkaline 

 rocks it is interesting to note the chemical composition of some 

 chert} 7 inclusions of metamorphic rock which are abundant in the 

 arfvedsonite trachyte of Timor Rock in the Warrumbungle 

 Mountains. These are referred to in my paper.* These inclu- 

 sions are rounded or subangular with rounded edges. The 

 trachyte gets more and more fine-grained as the inclusions are 

 approached and at the same time gets richer in blue soda amphi- 

 bole. In the following table the compositions of the host-rock 

 and of the inclusions are given, and it will be readily noticeable 

 that the difference between them is exceedingly slight, chiefly 

 noticeable in the MgO. It is not only possible but extremely 

 likely that the arfvedsonite trachyte actually represents this 

 sedimentary metamorphic rock (chert, adinole or halleflinta) 

 refused by some deep-seated process. 





Inclusion. 



Timor Trachyte 



wo a 



... 66-90 



65-90 



A1 2 3 



... 1575 



1674 



Fe 2 3 



FeO 



"• J 4-42 



172 i 391 



i-99 r dyi 



MgO 



... 1-04 



006 



CaO 



... 0-48 



0-09 



Na 2 



... 4-57 



6-35 



K 2 



... 5-18 



5-77 



H 2 0- 



H 2 0+ 



"• J 104 



°' 27 1 0-70 

 0-43/ U/U 



Ti0 2 



... 0-62 



0-25 



Zr0 2 



100-00 



29 





100 09 



The differentiation of the Warrumbungle and Nandewar rocks 

 has already been discussed in my papers dealing with those 

 regions. It need only be mentioned that the parent-magmas of 

 the two regions are very closely related chemically. 



It might be objected to this ' refusion and assimilation 

 theory ' which I suggest to explain the origin of alkaline rocks 



* Proc. Linn. Soc. N.S. Wales, 1907, Vol.xxxii. pp. 560 and 595. 



