BY H. I. JENSEN. G01 



F. Chemical Evidences of Genetic Relationship. 



I have, in previous papers, given analyses of trachytic rocks 

 from the various alkaline areas of Australia. A comparison of 

 these analyses and of those of their basic associates is invited. 



The points of affinity which are most apparent in the chemical 

 constitution of all the Australian alkaline rocks may be sum- 

 marised as follows : — 



1. They are invariably very rich in alkali. 



2. Soda and potash occur in about equal molecular proportions, 

 sometimes one, sometimes the other slightly preponderating in 

 amount. The effect of nearly equal proportions of these two 

 radicals is to produce anorthoclase, with the slight excess of soda 

 going into the aegirine. 



3. Lime and magnesia are very deficient, particularly the latter. 

 This applies to the acid, intermediate, and basic rocks alike. In 

 the more basic rock-types lime, of course, increases in amount, but 

 is low for the silica-percentage, and magnesia is still lower pro- 

 portionately. This statement, however, does not apply to 

 lamprophyric (monchi quite) dyke-rocks. 



4. The iron oxides vary inversely with the basicity, and our 

 alkaline rocks are all characterised by a high iron-percentage for 

 their basicity. The ferrous and ferric oxides occur in nearly 

 equal proportions in most cases, the higher oxide preponderating 

 in the aegirine rocks. 



5. Titanium dioxide is rather high in all varieties, increasing 

 in amount with increase of basicity of the rock. Phosphoric 

 anhydride is correspondingly low. Zirconia is very frequently 

 present. 



6. Increase of CaO affects the character of the pyroxene only, 

 unless the rock be moderately or very rich in CaO. 



7. A1 2 3 is high, as is to be expected from the abundance of 

 felspar. 



8. In the Queensland areas there is a distinct tendency for 

 Fe 2 3 to greatly exceed FeO in amount. This is ascribed to — 

 (1) Riebeckite and aegirine primarily dominating over arfved- 

 sonite. (2) Fe 2 3 has separated as haematite and limonite by 



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