﻿"Vol. 66.] AND DACITES OF THE DA.NDEN0NO DISTRICT. 461 



gneiss, while clusters of biotite and quartz are occasionally so 

 arranged as to suggest by their shape the outlines of hypersthene 

 phenocrysts. It appears, therefore, to be the case that in these 

 gneisses we are dealing with the limiting stages of mineralogical 

 change, the interesting earlier stages of which are represented by the 

 altered dacites already described. The rock shows a marked parallel 

 orientation of the minerals, and this is accompanied by certain 

 strain-polarization effects. Some of the felspar phenocrysts show 

 granulitization at their extremities, and a few now consist of 

 aggregates of secondary felspar which fairly preserve the original 

 shapes of the phenocrysts. Combined with these changes I often 

 noticed the development of secondary granules within the felspar 

 phenocrysts and a criss-cross aggregation of the biotite, which allies 

 these rocks with the altered dacites. 



V. Chemical Composition of the Minerals and Rocks. 



Investigation of the chemical composition of the minerals of the 

 dacite and bulk analyses of the dacite, the gneiss, and the grano- 

 diorite were undertaken, in the hope that they would throw light 

 on two problems : 



(1) The relations existing between the dacite, the gneiss, and the grano- 



diorite ; and 



(2) The evidence as to the conversion of original minerals in the dacite into 



the secondary minerals of the altered dacites. 



In 1906 two of my students, Mr. Plante and Mr. Richards, 

 at my suggestion made chemical analyses of the normal dacite, 

 the gneiss, and the granodiorite. Their results showed that a 

 very close similarity in composition exists between the three rock 

 types. The analyses were, however, not entirely satisfactory, since 

 no determination was made of the alkalies present in the reagents 

 used, and in consequence too high a percentage of soda was 

 indicated in the rocks. As, however, all three analyses were made 

 under similar conditions, they are useful for purposes of comparison. 

 They show in each case that the percentages of alkalies and 

 alkaline earths are practically identical, and that the granodiorite 

 has rather more silica and less total iron-oxides than either the 

 dacite or the gneiss. The differences are, however, so slight, and 

 the resemblances so close, that all the rocks may confidently be 

 referred to a common magma-reservoir and to the same geological 

 period. 



Mr. Richards's later chemical work (13) is of considerable 

 importance, and includes analyses of the normal dacite and 

 of each of its ferromagnesian minerals. His results are as 

 follows : — 



