﻿Yol. 66.^\ AND DACITES OF THE DANDENONG DISTRICT. 459 



Biotite after hypersthene. — Among the altered dacites 

 near the granodiorite contact, secondary biotite is sometimes formed 

 at the expense of the original hypersthene. 



Slide No. 458 (PI. XXXIII, fig. 1) shows a large phenocryst 

 of hypersthene, the margin of which is now frayed and ragged. 

 Immediately surrounding the hypersthene are numerous minute 

 flakes of secondary biotite derived from the hypersthene. 



Secondary quartz from hypersthene. — In the same slide 

 some clear grains of quartz associated with the biotite appear also 

 to be secondary and to be derived from the hypersthene. 



Enstatite and bastite after hypersthene. — In some 

 cases near the contact the alteration of the hypersthene has pro- 

 ceeded differently. Iron has been transferred from the central to 

 the marginal parts of the crystal. The result has been the 

 formation of enstatite, often altered to fibrous biotite, and the 

 deposition of haematite or of secondary biotite round the margin of 

 the crystal. The conversion of hypersthene into enstatite and bastite 

 is a change commonly attributed to weathering, and it is quite 

 possibly due to that cause in this case. 



Secondary tourmaline. — In many of the altered dacites 

 near the granodiorite contact tourmaline has been recognized. 



No. 812 shows in the centre of the field a minute elongated 

 brown crystal which has a high refractive index, no cleavage 

 visible, and intense pleochroism, the maximum absorption occur- 

 ring in a position at right angles to that characteristic of the 

 biotite. The mineral shows straight extinction, is uniaxial and 

 negative, and there can be no doubt that it is tourmaline. This is 

 the only section in which normal brown tourmaline has been 

 noticed in an altered dacite. Quite a considerable number of 

 slides of the altered dacites contain aggregates of very minute 

 and generally irregular granules of a bluish-grey pleochroic mineral. 

 This I at first regarded as a soda-hornblende, and have referred to 

 it as such (14). Later and more detailed examination shows that a 

 number of the granules are hexagonal in outline, and one or two 

 elongated- sections were seen. These latter show straight extinc- 

 tion, strong pleochroism, absorption in a position at right angles to 

 that shown by biotite, and are optically negative. These characters, 

 together with the hexagonal shape of some of the sections, indicate 

 that the mineral is a peculiar variety of blue tourmaline. It is to 

 be noted — and this is well seen in such sections as No. 465 

 (PI. XXXIII, fig. 2) — that this blue tourmaline is nearly always 

 closely associated with hypersthene and less commonly with biotite. 

 In No. 465 clusters of tourmaline occur at both ends of a corroded 

 hypersthene phenocryst. This I regard as indicating that the 

 mineral is secondary in origin, and formed by the reaction of 



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