﻿468 PROF. E. W. SKEATS ON THE GNEISSES [Allg. I9IO,. 



EXPLANATION OF PLATES XXXII-XXXIV. 



[The numbers refer to the slides in the Geological Collection of the 

 University of Melbourne.] 



Plate XXXII. 

 Fig. 1. No. 71. Magnified 25 diameters (ordinary light). — Biotite-hypersthene- 

 dacite from Mount Dandenong, showing phenocrysts of plagioclase, 

 hypersthene, biotite, and ilmenite in a macrocrystalline ground-mass 

 of quartz, felspar, and biotite. (See pp. 457-58.) 

 2. No. 418. Magnified 25 diameters (ordinary light).— Altered dacite 

 (slightly schistose), 100 yards from the contact with the granodiorite, 

 south of Selby. The section shows criss-cross aggregates of secondary 

 biotite, formed bv the reaction of ilmenite with the ground-mass. 

 (See p. 458.) 



Plate XXXIII. 



Fig. 1. No. 458. Magnified 106 diameters (ordinary light).— Banded dacite, 

 Monbulk Creek. The section shows the corrosion of a phenocryst of 

 hypersthene, with marginal formation of secondary biotite, quartz, 

 and blue tourmaline. (See p. 459.) 

 2. No. 465. Magnified 106 diameters (ordinary light;. — Altered dacite, 

 south of Up way, near the contact with the granodiorite. Note the 

 corrosion of the phenocryst of hypersthene and the formation at its 

 extremities of secondaiy biotite, quartz, and a considerable amount 

 of blue tourmaline. (See p. 459.) 



Plate XXXIV. 



Fig. 1. No. 426. Magnified 25 diameters (ordinary light). — Altered and 

 schistose dacite, near the contact with the granodiorite south of Selby. 

 Note the parallel orientation of the minerals and the fact that the 

 big phenocryst of hypersthene is largely altered to secondary biotite 

 and quartz. (See p. 460.) 

 2. No. 371. Magnified 25 diameters (ordinary light).— Gneiss from the 

 upper part of Monbulk Creek. Note the absence of hypersthene and 

 ilmenite from the rock, as also the well-developed foliation which it 

 exhibits. (See p. 460.) 



Discussion. 



Dr. A. P. Young noted the marked contrast in structure between 

 the effusive and the intrusive rocks, leading to the conclusion that 

 the former had accumulated to a great depth before the iutrusion 

 took place. In the scanty proportion of dark constituents, the* 

 vein-like occurrences in the dacites resembled aplites. It would 

 be interesting to know the composition of the felspars and to learn 

 how far the relative proportions of lime-, soda-, and potash- 

 felspars obtaining in the granodiorite had been maintained. 



The parallel structure of the dacite recalled the fine banding 

 often seen in andesitic rocks elsewhere, notably in those of Ecuador 

 so well shown in the collections of Dr. W. Reiss & Dr. A. Stubel. 

 In this case, however, the characters in question had been acquired 

 at first hand in the course of consolidation, and it seemed im- 

 possible to interpret them as contact-effects due to later intrusions. 



In the Klausen district of Tyrol rocks of a dioritic series broke 

 through the 'Feldstein,' which was much older than the diorite 



