and their Contact Zones. 45 



In many cases, and especially in those quartz diorites in 

 which magnesia mica is a constituent, I have observed, inter- 

 polated among the mica plates, mineral aggregates of a pale- 

 yellow or brownish-yellow colour. They have a somewhat 

 rough appearance of surface, and are sensibly dichroic. 

 They show minute aggregate polarisation in bright shades — 

 mostly greens and reds. It seems to me most probable that 

 this mineral is an alteration product. I have found similar 

 aggregates filling in spaces or adjoining the felspars, in one 

 case surrounding hornblende. It has suggested itself to me 

 that this may be epidote, or in some cases even epidosite. 



Classification of the Igneous Rocks. 



All these rocks are intrusive, and in nowise contem- 

 poraneous. The following classification may be made : — 



Granites. — This group is confined to the eastern part of 

 the intrusive area, as, for instance, the Long Gully and the 

 Sheep Station Creek. Its members are all of a quartzose 

 character. The predominant felspar is orthoclase, which is 

 usually in crystalline masses, but is also met with crystal- 

 lised. Potash felspar also occurs in its triclinic form micro- 

 cline. Plagioclase is always more or less present in well- 

 formed prismatic crystals. Magnesia mica is very constantly 

 present, but is occasionally almost wholly replaced by horn- 

 blende, while elsewhere both are found together. Here, 

 then, we have granitite as the prevailing rocks of this class, 

 and also an amphibole granite with varieties connecting the 

 two. Where a large amount of plagioclase is present, there 

 is a passage to the adjoining quartz diorites. 



Quartz Diorites. — Rocks of this class are found adjoining 

 the granitites, as at Riley's Creek, or in the outer parts of 

 the intrusive mass, as at Eureka and the upper part of 

 Swift's Creek. Quartz is often a marked constituent, in 

 which case the rock assumes either a strongly gneissic or 

 granitic character. It is most usual, however, for the tri- 

 clinic felspars to predominate ; and out of twenty-four slices 

 which I selected as representing this class (all of them con- 

 taining quartz as an essential constituent) I found that one 

 contained felspars of group 1 ; five of groups 1 and 2 ; two 

 of group 2 ; one of groups 2, 3, and 4 ; and sixteen of 

 groups 1, 3, and 4. I have to remark again that the most 

 quartzose rocks are those which contain the potash and soda 

 felspars; while those which contain the lime felspars are 

 almost or wholly quartzless. 



