270 N. R. Junner: 



varied conditions, strongly suggests that it is a primary mineral. 

 Although only rarely visible macroscopically in the rocks from the 

 Healesville area, microscopic examination has revealed its presence 

 in granodiorites, dacites. rhyolites, and tuffs. Its occurrence in 

 these rocks (both plutonic and extrusive) certainly suggests that 

 it was an original mineral in the magma from which these rocks 

 were derived. Microscopical examination of these garnet-bearing 

 rocks, however, leads one to doubt the primary origin of the 

 garnet for the following reasons : — 



1. The apparently homogeneous crystals of garnet, occurring in 

 hand specimens of the rocks, are seen under the microscope to b« 

 aggregates of garnet grains associated with secondary quartz, 

 pyrrhotite, chlorite after biotite, sericitised felspar, and in on» 

 case with blue tourmaline. 



2. The association of garnet with pyrrhotite and chloritised 

 biotite is characteristic. Rims of pennine occasionally surround 

 the garnet aggregates. Pyrrhotite is often a contact metamorphic 

 mineral, and has been noticed in the altered sediments adjoining 

 granodiorite at the Maroondah aqueduct. 



3. Garnet is apparently absent in the hypersthene bearing 

 dacites; at least this is so in all the sections of these rocks that 

 I have examined. Sir Thomas Holland has described certain rocks 

 in Avhich garnet apparently replaces hypersthene, but in otir case 

 sufficient evidence is not available to determine whether such 

 replacement has taken place. 



Contact metamorphism of the volcanic rocks only becomes marked 

 near certain of the granodiorite intrusions, and garnet is invari- 

 ably absent from the contact rocks, so that it is improbable that tlie 

 garnet is a contact metamorphic mineral. 



Abyssal magmatic assimilation might possibly account for the 

 presence of the garnet in the Healesville igneous rocks. Cordieriie, 

 sillimanite and garnet have been previously mentioned as forming 

 at Lipari owing to the assimilation of an andalusite bearing rock 

 by an andesite. 



Summing up, no definite conclusion, respecting the origin of the 

 garnet, can be arrived at, and it is left for future investigators to 

 make a more detailed study of the question. 



(J . — Andesifex. 

 Quartz free andesites are rarely met with \u the Healesville area, 

 Tlie best example of these rocks seen by the writer occurs at a point 



1 "Geology of the iifii;hl)oiiflioo(l of Salem, etc' Moinoir,^ (Jfol. Suiv, Inrlia, vol. x.\x., 1900 



