Victorian Igneous Rocks. 283 



widely from the curve, l)i)t that the hleiid of the analyses of these 

 rocks agrees closely with the curve. 



In dealing with the differentiation of the Macedon alkali series 

 the assumption was made that as the lavas had in general been 

 poured over a fairly level plain, that their bulk would be approxi- 

 mately proportional to the areas occupied by them. 



Working on the above assumption the average composition tjf 

 the alkali rocks Avas determined, and is given as No. 11 in Tables 

 XII. and XIII. 



After the extrusion of the more acid types it was estimated that 

 the composition of tlie residual magma would be approximately 

 that given as No. 6 in Tables XII. and XIII. 



It will be seen that this composition agrees very closely with 

 that obtained by blending the so-called complementary types in 

 proportion to area, and also with that of the anorthoclase olivine 

 trachyte from Sugarloaf Hill. Tliis suggests that after the extru- 

 sion of the solvsbergite and acid tiachyte the residual magma split 

 into two parts of similar composition, one of which gave rise to the 

 anorthoclase olivine trachyte and limburgite, and the other portion 

 split up to form the macedonite, woodendite and anorthoclase basalt. 



It has been suggested that the differentiation of the alkali rocks 

 may be subdivided into two forms — (1) Serial Differentiation, (2) 

 Complementary Differentiation. To the Serial differentiation are 

 assigned those types which conform to the curves, and w'hich shoAv 

 a regular gradation in the percentage of the various constituent 

 oxides. The remaining types show no such regularity in their 

 variation, but appear to have been produced by an irregular sub- 

 division of a partial magma, and consequently are considered to 

 have been derived by complementary differentiation. 



The serial differentiation of these alkali rocks may conveniently 

 be referred to as curvilinear serial. 



The probable limiting values of the curvilinear type of serial 

 differentiation cannot be determined sufficiently accurately to be of 

 any value, owing to the difficulty of satisfactorily producing the 

 curves. l)ut judging from the norms of the solvsbergites and lim- 

 burgites the acid limit value would be composed of quartz and 

 anorthoclase, and the basic limit value of anorthite and olivine 

 with a concentration of magnetite, ilmenite and apatite. 



The relationship of the Macedon alkali series to other Victorian 

 alkali rocks and to the Newer Basalts has been discussed elsewhere 

 ^19). It was pointed out that the alkali lavas and the basaltic lavas 

 in the Macedon area occur in such close juxtaposition and were 



