Dacite and Granodiorite, 135 



:such fracturing occurring between a dome-shaped mass of volcanic 

 rock, and the platform on which it rests. 



Turning to the hypothesis of overhead or magmatic stoping* it is 

 found that this hypothesis is capable of giving a reasonable explanation 

 .of the occurrences under discussion. 



The original magma before the extrusion of the volcanic phase, by 

 means of overhead stoping reached sufficiently near the surface for 

 partial collapse of the batholithic roof to take place, or alternately 

 for the production of fissures connecting the batholithic chamber with 

 the surface. In either case escape to* the surface of the upper portion 

 of the magma could take place. 



In the Macedon area there is no evidence of the occurrence of any 

 pyroclastic material so that the extrusion was not accompanied by any 

 (explosive action. The lava must have been fairly viscous, as the 

 dacite was piled up in a dome-shaped mass, and wide spread lava 

 flows are not found. A small flow occurs forming a tongue to the 

 south west of Upper Macedon. At Healesville, and near Lilydale, in 

 the Mt. Dandenong area, pyroclastic material has been recorded, in- 

 dicating that conditions were somewhat different in those areas. 

 Dome-shaped masses of dacite are also found in the Dandenong and 

 Healesville areas. 



Solidification of the dacite sealed up the vents from the batholithic 

 chamber, and conditions became favourable for the resumption of 

 stoping, with the result that the palaeozoic platform was entirely 

 removed from a considerable area, bringing the granodiorite in direct 

 contact with the dacite. If this explanation be correct then we might 

 expect to find in some area portion of the old platform which had 

 not been entirely removed, and which should occur between the 

 dacite and granodiorite. So far no such occurrence has been recorded, 

 but may exist, and further field work along the contacts should 

 T^e done. 



The dacites and granodiorites are closely related chemically, but 

 exhibit distinct mineralogical differences. Near Braemar House (now 

 called Clyde) in the Macedon district, both these rocks occur, and 

 analyses are given in the following table. A complete list of the 

 analyses is not necessary as all have been recorded, together with 

 variation diagrams, in the publications quoted on the first page of 

 this paper. 



The typical dacite of Macedon consist of phenocrysts of labra- 

 dorite and hypersthene with smaller and less numerous phenocrysts 

 of ilmenite and biotite set in a granulitic groundmass, consisting of 

 quartz, orthoclase, plagioclase, biotite and ilmenite. 



The phenocrysts of labradorite and hypersthene generally show 

 corrosion, but this is more marked in the pyroxene than in the felspar. 

 Biotite frequently occurs in aggregates bordering the hypersthene. 

 The granodiorites consist of quartz, labradorite, orthoclase, and biotite, 

 the ratio of plagioclase to orthoclase being nearly 6 to 1. The prin- 



4. Btaly, R. A., Am. Jour, of Sc, Vol. 15, p. 269, 1903. 

 Igneous Rocks and their origin, 1914. 



