﻿Vol. 66, ,] AND DACITES OF THE DANDENONG DISTRICT. 467 



of foliated and gneissic rocks which fringe the contact; for several 

 miles. In some of the microscope-slides a marked schistose or 

 gneissic structure is described as an effect produced by differential 

 movement in the rock, combined with features due to contact- 

 metamorphism. Through the courtesy of Dr. J. S. Flett, to whom 

 I am also indebted for several helpful suggestions, I have had an 

 opportunity of examining these slides, and in a general way the 

 structures shown resemble those in the gneissic rocks which I have 

 described above. 



Until further evidence is available, it would be unwise to say 

 more than that the weight of evidence at present seems to be 

 in favour of regarding the true gneisses as rocks which were 

 originally dacites, and owe their special structural and mineralogical 

 characters mainly to differential movement and in a subsidiary way 

 to later thermal metamorphism. 



VIII. Summary and Conclusions. 



The area described lies about 20 or 25 miles south-south-east of 

 Melbourne. By the earlier surveyors the dacites were described as 

 traps of Palaeozoic age passing gradually into the granites. Prof. 

 Gregory has described them as dacites, and as probably of Lower 

 Tertiary age and resting upon the denuded surface of the grano- 

 diorite. I believe that large masses of hypersthene-biotite-dacites 

 were poured over and also intruded into Silurian or Ordovician 

 sediments, probably in the Devonian Period. At a slightly later 

 date, and from the same magmatic reservoir, granodiorite was 

 intruded into the sediments and the dacites. Along the contact 

 with the dacites acid veins are developed, and altered and gneissic 

 modifications of the dacite occur. The characters of the dacite, 

 granodiorite, and schistose and gneissic rocks under the microscope 

 are described. 



In the contact-rocks, hypersthene and ilmenite in particular pass 

 over into secondary biotite and quartz and secondary biotite 

 respectively, while blue tourmaline is also found. The chemical 

 compositions of these rocks and of the ferromagnesian minerals are 

 given, and it is shown that their chemical compositions are in 

 accord with the observed mineralogical changes. These effects, 

 together with the production of a slight banding or schistosity, are 

 referred to the contact-metamorphism produced by the intrusion 

 of the granodiorite, while the origin and distribution of the true 

 gneissic dacites is a more difficult problem. This production was 

 formerly referred to contact-metamorphism alone, but now, partly 

 owing to peculiarities of distribution and partly to evidence of 

 crushing, these rocks are tentatively thought to be due mainly to 

 differential movements in the dacite before the intrusion of the 

 granodiorite, complicated by mineralogical and structural changes 

 due to its intrusion. 



