Ml IK' nils ill JJdcife. 535 



Comments on the Analyses. 



Dacite. — The coinposition of this dacite is seen to be quite 

 normal, except that perhaps the TiO.^ percentage is a little 

 higher than usual in rocks of this type. A comparison of this 

 analysis with one of a dacite^ from near Braemar House, Mount 

 Macedon. shows that the two are almost identical. In com- 

 position this rock miuli resenibles a (]iKirtz-niica-diorite from 

 EnsMy,'- Omeo, analysed by the late Dr. Howitt, and it is 

 interesting to note that all three fall into the same sub-rang 

 when classified according to the American classification — 

 viz. : — ■ 



Class II. --Dosalane. 



Order IV. — (^)unrdofelic, Anstrare. 



Rang III. -Alkali-calcic, Tonalase. 



Sul)rang III. — Sodipotassic, Harzose. 



The Ensay rock, however, nearly passes into subrang iv., while 

 both the dacites are well in subrang iii. 



Biot,ite.— 'Y\\e Si().„ FeO and TiO.^ percentages are higli, 

 while the alumina and alkali percentages are low. This mineral 

 in thin sections, under the microscope shows inclusions of ilme- 

 nite, thus the TiO.^ and FeO percentages are too high, while all 

 the other constituents are correspondingly low. The biotite 

 itself is believed to be titaniferrous, so that it is difficult to 

 know how much TiO._, to assign to the biotite and how much to 

 the enclosed ilmenite ; consequently no deduction was nuide from 

 the analysis for these two oxides, with the result tliat the RO^ 

 and RO ratios are a little too high. 



The formula for this mineral, according to the analysis, is 

 2R.p.4RO.R.A-6RO.,. 80 tliat it is not ;in ortliosilicate 

 like the typical biotites. 



Hypersthene. — In this analysis the FeO percentage is slightly 

 higher than usual, while the MgO percentage is considerably 

 lower than in a normal hypersthene, with the result that the 

 formula for this mineral is 3R0.4RO.,. 



1 Annual Report of the Secretary for Mines Victoria, 1!)07, p. 61. 



2 Proc. Ko.v. .Soc. Victoria, vol. xxii., p. 99. 



