﻿"Vol. 66.~] AND DACITES OF THE DANDENONG DISTRICT. 457 



The acid veins. — Several of the acid veins are almost free 

 from coloured minerals, and look like aplite. They have, however, 

 a hypidiomorphic to graphic and not a panidiomorphic structure. 

 They consist chiefly of alkali felspar, with a perthitic intergrowth of 

 albite and of quartz. A little acid oligoclase, biotite, and tourmaline 

 are also present. One true pegmatite vein occurs in the railway- 

 cutting near Belgrave Station, and consists of alkali and lime-soda 

 felspars, quartz, and tourmaline. The section shows a fairly good 

 intergrowth of quartz and felspar. In no case do any of these acid 

 veins exhibit any signs of strain or foliation, either in the field or 

 under the microscope. 



{b) The Dacite. 



As an example of a fairly normal dacite, the rock described by 

 -one of my assistants, Mr. H. C. Eichards (13), from a quarry just 

 west of TJpway Station, may be noted. The following account is 

 taken chiefly from Mr. Eichards's description. The dacite is a dark- 

 grey and very hard, compact, porphyritic rock. In the hand-specimen 

 phenocrysts of felspar and of a dark-brown biotite, in well-formed 

 hexagonal flakes, can be seen. Under the microscope in thin section 

 are seen abundant regular zoned phenocrysts of plagioclase, which 

 are mainly Ab 1 An 1 in composition, as determined by the Michel-Levy 

 method. Dark-brown biotite, pale-green hypersthene, occasionally 

 quartz, and opaque grains of ilmenite are also porphyritic con- 

 stituents. Some ilmenite occurs as inclusions both in the hypersthene 

 and in the biotite. In places ilmenite grains are surrounded by 

 narrow fringes of biotite, in such a way as to suggest that before 

 the ground-mass crystallized under superficial conditions reaction 

 occurred between the ilmenite and the acid ground-mass, giving 

 rise to a fringe of secondary biotite. The phenocrysts are set in a 

 fine-grained microcrystalline ground-mass of quartz, felspar, and 

 biotite. The felspar of the ground-mass appears to be a mixture 

 of orthoclase and of plagioclase of the composition Ab 9 An 8 . 

 Phenocrysts and ground-mass are about equally represented. The 

 average diameter of the phenocrysts is about 0*1 millimetre, 

 and of the minerals of the ground-mass only about O'Ol mm. 

 Mr. Eichards's calculation of the mode of the rock by Eosiwal's 

 method yielded the following result : — 



Mineral percentage. 



f Plagioclase (Ab x Ai^) 2413 



| Hypersthene 1217 



Phenocrysts \ Biotite 1096 



| Quartz 122 



^Ilmenite 108 



[ Orthoclase | 



(Felspar pla - d olas L 22-95 



Ground-mass-j |_ (Ab 9 An 8 ). J 



| Quartz 2034 



^ Biotite 715 



Total 10000 



Q. J. G. S. No. 262. 2 i 



