55 



apart from this the mineral is well distributed through the 

 rock as a rule, and included in all the other constituents, 

 both of the hornfels proper and of the quartz-chlorite veins. 

 Zircon, too is fairly abundant, and in some cases apatite. 

 The former shows up well in biotite and chlorite by reason 

 of the haloes and coronae which envelop it. One effect of 

 the zircon on its host is to cause the part of it covered by the 

 halo to have a slightly weaker birefringence than the rest. 



As regards chlorite, that in the veinlets is identical with 

 what is found in the albite syenite, and in addition to the 

 bladed forms appears as broader somewhat irregular grains, 

 while the fine-grained material of the hornfels proper is very 

 pale, almost colourless, and often crystallized, vertical sections 

 giving lath-like shapes. The derivation of this chlorite from 

 biotite is proved by the occurrence of occasional crystals of 

 biotite of exactly the same habit, which is in striking con- 

 trast with the entirely irregular forms characterizing the 

 micaceous quartzites, and resembles most the biotite of the 

 strongly schistose rocks. 



From the prevalence of albite and rutile in all these 

 rocks it is evident that there has been an important transfer 

 of material from the syenite magma, especially of soda and 

 titania, producing a rutile-bearing albitic hornfels as a result 

 of metasomatic metamorphism. It seems as if one must pos- 

 tulate two stages of alteration to account for all the effects 

 produced. The albite syenite was first injected, with accom- 

 panying veins and veinlets, the country rock being impreg- 

 nated with albite, and to a less degree, with rutile, zircon, and 

 apatite. Subsequently circulating solutions, still magmatic in 

 character, attacked the biotite of both igneous^ rock and 

 schists, converting it into chlorite. 



The presence of chlorite in the knotted schists closely 

 associated with the porphyroblasts suggests that the circulat- 

 ing solutions had probably something to do with the production 

 of the minerals forming the 'knots," while the peculiar feat- 

 ures mentioned in connection with the sericite aggregates 

 suggest that these may possibly represent an intermediate 

 stage in the production of the andalusite. 



VIII. Summary. 

 (1) A group of igneous rocks at Encounter Bay is described, 

 comprising two related series : (a) A plutonic series consisting: 

 of quartz mica diorite, porphyritic granite, even-grained 

 granite, and albite mica syenite ; and (h) a series of minor 

 intrusions consisting of uralitic dolerites, potash aplite, and 

 soda aplite and granophyre. 



