202 Proceedings of the Royal Society of Victoria. 



VI. — The Relations of the Macedon Rocks. 



After the foregoing account of the rocks of the Macedon 

 series, we may proceed to consider their general petrographic 

 relations. In the first place it is clear that the rocks, excluding 

 the grano-diorite, are part of one petrographic series, having 

 been formed by difierentiation from one magma. They all 

 belong to the intermediate group, and are characterised by a 

 high percentage of soda. The three rocks that have been 

 analysed vary from 62-7 to 65-9 % of silica, and from 9-79 to 

 13-46 % of alkalies. The soda varies from 6-28 % in Mr. Lewis's 

 analysis of the Willimigongong rock to 10-1 % in Newbery's 

 analysis of the rock of the Camel's Hump. The microscopic 

 examination, showing the predominance in the superficial member 

 of the series of anorthoclase, aegerine, nosean, riebeckite and 

 cossyrite (aenigmatite), agrees with the chemical analysis, and 

 shows the abundance of soda in the rock. 



That the grano-diorites are not members of the Macedon series 

 is shown by the absence of passage rocks, and of the typical 

 minerals of the Macedon group from the diorites. The Macedon 

 effusive rocks are not those that would have been formed by 

 eruption from the grano-diorites that underlie them on their south- 

 eastern margin. The plutouic representative of the Macedon 

 lavas is probably a nepheline-diorite, which has not been found 

 in Victoria. 



The member of the Macedon series which originated at the 

 greatest depth is the Willimigongong type of dacite. It occurs 

 in big intrusive dykes or masses along the Willimigongong, at 

 Cheniston, Towrong, Braemar, Cherokee, etc. ; in fact wherever 

 denudation has cut at all deeply into the Macedon block. 



The jnicroscopic examination of this rock at first suggests for 

 it the name of hypersthene-andesite, as hypersthene and a fairly 

 basic plagioclase are the two most striking constituents. The 

 abundance of free quai-tz in some sections, however, necessitates 

 its inclusion among the dacites. The term hypersthene-dacite 

 would, however, alone be inadequate. The facts that the rock 

 is holocrystalline and that it occurs in intrusive dykes are not 

 final objections, for Rosenbusch accepts (17, vol. i., p. 450) a 

 group of holocrystalline dacites. The most serious objection is 



