MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 193 



of plagioclase and a monoclinic pyroxene. It was supposed, at 

 one time, that hornblende characterised ancient rocks, and that 

 the presence of augite could be relied on as distinguishing rocks 

 of a more recent date. " It may be admitted that this distinction 

 is a real one, but its significance and value are greatly diminished 

 when we remember the relations which exist between the two 

 minerals in question. Hornblende and augite are interesting 

 examples of a dimorphous substance ; in chemical composition 

 they are identical, or rather they are liable to variations between 

 the same limits, but in their crystalline forms and optical charac- 

 ters they differ from one another. It has been proved that horn- 

 blende is the stable, and augite the unstable condition of the 

 substance in question. If hornblende be fused and allowed to 

 cool it crystallises in the form of augite. On the other hand, 

 augite-crystals in rocks of ancient date are found undergoing 

 gradual change and passing into hornblende."* 



As far as I have been able to observe in this colony the presence 

 of hornblende, rather than augite in an eruptive rock is no indi- 

 cation of its age. Neither have I so far been able to point to an 

 undoubted change from a rhombic pyroxene to a mineral optically 

 monoclinic, nor even of an augite to a hornblende. These inter- 

 esting alterations have been noted both in American and European 

 rocks, and will no doubt be observed in Australian rocks as the 

 study of petrology becomes popularised. f 



Classification of the Rocks. 



As a matter of convenience it will be well to group the rocks 



dealt with. I shall therefore take the material after this scheme. 



Eruptive Rocks. 



I Granite. 

 Holocrystalline. 1 Hornblende-biotite granite. 



Acidic.-^ ( Biotite granite. 



Micro -crystalline, or j Quartz porphyry. 

 Crystalline with a paste < Felspar porphyry. 

 ( Quartz felsite. 



* Volcanos, by J. W. Judd, f.r.s., 1881, p. 267. 



f Williams, Gabbros of Baltimore — Bulletin of U. S. Geol. Survey, 

 No. 28, p. 47. 



M-Oetober 7, 1891. 



