﻿Vol. 66.] AND DACITE8 OF THE DANDENONG DISTRICT. 463 



the observed passage of hypersthene into secondary biotite and 

 secondary quartz is in harmony with the known chemical com- 

 position of the minerals present in the parent rock. 



(ii) The addition of orthoclase to the ilmenite will give a 

 mixture resembling in composition a biotite very low in magnesia 

 and very rich in ferrous oxide. The observed optical characters of 

 the secondary biotite fringing the ilmenite are somewhat different 

 from those of the primary biotite phenocrysts, and suggest that the 

 change has taken place on the lines stated above. 



The general results obtained from an examination of the chemical 

 composition of the three rock-types and of the minerals of the dacite 

 are therefore : — 



(1) The granodiorites, gneiss, and normal dacite are 

 all the products, very slightly differentiated, of a 

 common magma-reservoir, and they all belong approxi- 

 mately to the same geological period (probably the 

 Lower Devonian). 



(2) The secondary minerals mentioned above have 

 been derived from the alteration of, and reaction 

 between, the primary minerals of the dacite. 



VI. Effects produced by Contact-Metamorphism. 



The field-evidence for the intrusion of the granodiorite into the 

 dacite series has already been summarized. The sharp junction, 

 the acid veins penetrating the dacites, and the alteration in appear- 

 ance of the dacites near the contact serve to establish this point. 

 The evidence of thin sections adduced in the foregoing pages shows 

 that the dacites near the contact exhibit definite mineralogica 

 changes, while the chemical evidence also supports these conclusions. 

 The question of the origin of the gneiss will be discussed later ; but 

 the other alterations in the dacite near the contact may be confidently 

 regarded as the results of thermal or contact-metamorphism. 



These changes may be summarized as follows : — 



(1) The constant corrosion of primary biotite by the ground-mass with 



marginal flaking-off of small biotite crystals. 



(2) The production of marginal fringes of secondary biotite surrounding the 



original ilmenite. 



(3) The partial alteration of hypersthene to secondary biotite and secondary 



quartz. 



(4) The reaction between ilmenite and the felspar of the ground-mass, 



leading to the formation of secondary biotite. 

 (T>) The formation of minute grains of blue tourmaline, noticed only in the 



dacite near the granodiorite contact. 

 (6) The production of a subparallel orientation of the minerals in the dacite, 



giving rise to a slight banded or schistose tendency in the rock. 



The literature dealing with the effects of the intrusion of plutonic 

 into volcanic rocks is not copious. To some of this I have but 



