﻿Vol. 66.] AXD DACITES OF THE DANDENONG DISTRICT. 465 



biotite, and magnetite dust. The mica clusters round phenocrysts,. 

 which are corroded. The enstatite is surrounded by pyroxene 

 granules, biotite flakes, and magnetite dust, which also occur in 

 cracks in the crystals. In the rocks near the granite the flakes of 

 biotite are larger, and cluster round the margins of the porphyritic 

 pyroxenes. The enstatite, where changed to fibrous pseudomorphs, 

 shows secondary biotite and pyroxene in cracks. 



At the contact with the granite there is a great increase in the 

 development of secondary biotite. The enstatite sometimes is 

 altered to secondary biotite and magnetite with granular pyroxene, 

 sometimes to a green, fibrous, secondary amphibole. The former type 

 of alteration is from fresh enstatite, the latter when the enstatite 

 had first been changed to bastite. Banded rocks with a streaky 

 appearance occur in the altered zone, and Mr. Kynaston thinks 

 that they may be altered tuffs. 



It will be noted that the production of secondary biotite and its 

 aggregation into clusters or as fringes to primary minerals are- 

 features which both Mr. Kynaston and myself have noticed as 

 characteristic of the contact-zone in volcanic rocks of intermediate 

 composition. I have not seen the production of secondary granular 

 pyroxene nor of secondary hornblende, described by Mr. Kynaston. 

 Alterations noted by me, and not occurring in the rocks described by 

 Mr. Kynaston, are the corrosion of primary biotite, the production 

 of secondary quartz as one of the alteration -products of a rhombic 

 pyroxene, the production of secondary biotite from ilmenite, and 

 the presence of tourmaline in the altered rocks. 



VII. Origin of the Gneisses. 



Examination of these rocks in the field and in thin sections has 

 led me to describe them as being produced by extreme thermal 

 metamorphism of the dacite by the granodiorite (14). Further 

 work on this series has shown me that the evidence is not 

 sufficiently conclusive for a very definite statement as to their 

 mode of origin. The occurrence of the rocks is peculiar. The 

 acid granitic veins traversing the gneiss generally cut across the 

 foliation of the rock. The acid veins are quite unfoliated, and 

 show clearly that the production of the foliation was antecedent to 

 the intrusion of the acid veins. This fact led me to attribute the 

 foliation to contact-metamorphism. Difficulties occur, however, in 

 the distribution of the gneissic rocks. They occur in the Dandcnong 

 and in the Warburton areas, but have not been noticed at Maccdon, 

 Nyora, and other places where granodiorite is intruded into the dacite 

 series. Furthermore, as seen above, they only occur along part of 

 the line of contact in the Dandenong area. In other parts only 

 slightly schistose dacite, or (as south of Aura) a coarse-grained 

 slightly schistose dacite forms the contact-rock. South of Upway, 

 as seen in the sketch (fig. 3, p. 456), schistose dacite occurs at the 

 contact; and, on going northwards from this, alternations of normal 



