﻿458 PKOF. E. W. SKEA.TS ON THE GNEISSES [Allg. I9IO,. 



Some sections of the dacite exhibit flow-structure, but no signs of 

 strain or of the effects of subsequent movement in the rock are to- 

 be seen. PI. XXXII, fig. 1 illustrates the appearance of a normal 

 dacite in thin section. 



(c) The Altered Dacites. 



In these rocks, which are found only near the contact with the 

 granodiorite, a more or less definite parallelism in the arrangement 

 of the minerals is noticeable, and in the field they appear slightly 

 schistose. Under the microscope this is only sometimes noticed, but 

 important mineralogical changes, described below, are recognizable. 



Biotite after ilmenite (No. 418). — This may be taken as a 

 type of dacite in which the schistose structure is but feebly developed. 

 The same minerals occur as in the normal dacite ; but secondary 

 changes are common, and produce characteristic mineral aggregates. 

 PI. XXXII, fig. 2 shows the most noticeable feature, which is 

 the pronounced development of more or less circular clusters or 

 aggregates of secondary biotite, arranged in criss-cross fashion. 

 In examining a number of sections of the altered dacites all stages 

 can be traced, from that in which the biotite aggregates occur only 

 as a thin fringe to the ilmenite, through stages in which the biotite 

 is more developed, to the extreme case in which no ilmenite remains 

 and the cluster is composed entirely of biotite. In the case of this 

 rock, which occurs close to the granodiorite boundary, it is clear 

 that reaction between the ground-mass and the ilmenite has resulted 

 in the formation of clusters of secondary biotite. This differs 

 somewhat in appearance, habit, and optical behaviour from the 

 primary biotite phenocrysts, and suggests that the secondary is 

 probably different from the primary biotite in composition as well 

 as in mode of origin. 



Corrosion of biotite phenocrysts by the ground-mass 

 (No. 374). — This rock was taken from a quarry near Upway 

 Station, almost a mile from the granodiorite junction, but it 

 shows certain features more commonly seen in the rocks close 

 to the contact. Examination under a high power shows a large 

 phenocryst of biotite, which has been attacked at its margin by the 

 ground-mass. Small particles of biotite, which probably once 

 formed part of the original crystal, are now seen a short distance 

 from its margin ; while their place is now taken by minute quartz 

 and felspar granules, which appear to indent the edge of the pheno- 

 cryst. This effect is only seen occasionally in the normal dacite, in 

 which case it probably results from the corrosive action of the still 

 molten ground-mass on the biotite phenocryst, which was only 

 stable under deeper-seated conditions. It is, however, a feature 

 which is practically always present in the altered and slightly 

 banded dacites adjoining the granodiorite. 



