Igneous Rod's. 2(5'.) 



the margin of the hitter niineruL The garnet is seen under the 

 microscope to consist of irregular granular aggregates, resembling 

 the " siebstructure " of Weinschenck. Inclusions of biotite, 

 pyrrhotite and ilmenite are present in the garnet, and it is asso- 

 ciated with secondary quartz, zircon and tourmaline. A few lath- 

 ehaped sections are seen of a colourless mineral having moderately 

 high refractive index, but much less than that of zircon, and 

 exhibiting bright pinks and greens of the third order between 

 crossed nicols. The mineral shows straight extinction, and deter- 

 mination of its sign by a quartz wedge proves it to be positive. 

 It is, therefore, undoubtedly sillimanite, although its polarisation 

 colours are rather high for this mineral. Occasional laths of a 

 fibrous, strongly pleochroic (blue or violet to almost colourless) 

 tourmaline, having a negative sign and normal absorption, are 

 present in the section. Apatite and zircon are accessories. 



Section No. H73, dacite, from near Donnelly's Aveir, consists of 

 large fractured phenocrysts of colourless corroded quartz and fairly 

 large plagioclase crystals, showing both Carlsbad and xA.lbite twin- 

 ning, in a fine-grained pilotaxitic ground mass composed of labra- 

 dorite laths, chlorite and a little quartz. One or two vesicles, 

 infilled with chlorite and epidote, are present in this rock. Biotite 

 is rare, being greatly chloritLsed. 



Assimilation. — Assimilation of either aluminous sediments or 

 igneous rocks, has undoubtedly occurred to some extent, as is proved 

 by the presence of cordierite, sillimanite and garnet in certain of 

 these rocks. Numerous undigested sedimentary and igneous xeno- 

 liths are also seen in the dacites. 



A. Bergeatl has recently described an interesting case of the 

 melting up and assimilation of an andalusite bearing rock by an 

 andesite, with the formation of cordierite, sillimanite, garnet, 

 biotite, orthoclase and spinel. All these minerals, with the excep- 

 tion of the last named, are present in the Healesville dacites. 



Origiti of the garnet. — Pink garnet is relatively widespread in 

 (?) Devonian igneous rocks in Victoria. It occurs in granite or 

 granodiorite near Toora, Wilson's Promontory, Mount Taylor, 

 Beechworth, Healesville and elsewhere; and in intrusive and extru- 

 sive rocks, porphyries, porphyrites, daxites, etc., from near Mans- 

 field, Strathbogie ranges. Mount Dandenong, Healesville and War- 

 burton. The occurrence of the garnet in these rocks under such 



1 " Der cordieritaudesit von Lipari, seine Andalusit fiihreiiden einscliliisse mid die genetiselien 

 veziehungen zwisclien deni Andalusit, Silliinanit, Biotit, Cordierit, Orthoklas und Spinell in deni 

 letzteren." Neues. Jalieb. Min. Geol. (Beil. Bd.) .30, pp. 57.1-657, 1910. 



