﻿462 PROF. E. W. SKEATS ON THE GNEISSES [Aug. I9IO, 



A. B. C. D. E. 



Si0 2 6327 63-31 3986 50-42 



A1 2 3 16-50 14-44 11-13 4-06 



Fe 2 3 0-68 1-48 1-39 2-10 



FeO 510 4-66 1810 23-54 31-92 



MgO 2-48 2-35 988 13*)4 0"80 



CaO 4-18 3-67 sl.tr. P30 



Na 2 2-36 456? 035 trace 



K 2-68 2-67 673 0'69 



H 2 0+ 0-52 • ... 320 006 



H 2 0- 009 0-79 0-43 0-10 



C0 2 



Ti0 2 1-30 182 795 351 6728 



P 2 5 0-15 0-39 trace 0'92 



S(FeS 2 ) 0-16 



MnO 0-03 0-88? 0-58 0'24 trace 



Li 2 trace ... si. tr. 



Totals 99-50 1 0102 9960 99'98 10000 



Specific gravities... 276 ... 316 3*36 486 



A =; Dacite near Upway ; B = Gneiss, Monbulk Creek ; C = Biotite in dacite 

 from Upway ; D = Hypersthene in dacite from Upway ; E = Ilmenite in 

 dacite. 



The formulae of the minerals are as follows : — 



Biotite : 

 2R 2 . 4RO . E 2 3 . 6Si0 2 = 2(KH) 2 . 4(FeMg)0 . A1 2 3 . 6(SiTi)0 2 . 



Hypersthene : 

 12RO . R 2 0, . 16R0 2 = 12(FeMg)0 . (AlFe) 2 3 . 16Si0 2 . 



Ilmenite : 



RO . 2R0 2 = (FeMg)0 . 2Ti0 2 . 



The felspars in the rock determined optically were 



Phenocrysts — Plagioclase (Ab 1 An 1 ). 



Ground-mass — Orthoclase and plagioclase (Ab 9 An 8 ). 



With the foregoing evidence it is possible to see how far the 

 mineralogical changes observed under the microscope in the schistose 

 series can be confirmed by reference to the known composition of 

 the primary minerals in the rock. 



The most important mineralogical changes noted were 



(i) The alteration of hypersthene to secondary biotite and secondary 



quartz ; and 

 (ii) the reaction between ilmenite and the ground-mass, giving rise to 



secondary biotite. 



(i) On reference to the mineral analyses, it is seen that the 

 hypersthene, compared with the biotite, shows a marked excess of 

 silica, a deficiency in alumina, an excess of ferrous oxide and 

 magnesia, and a marked deficiency of potash. If an excess of 

 orthoclase, present in the ground-mass, be added to a molecule of 

 hypersthene, a mixture can be obtained bearing a close general 

 resemblance in composition to the biotite, with the exception that 

 a considerable excess of silica remains. It follows, then, that 



