32 Diorites and Granites of Swift's Creek, 



acquiring any other marked character. When the felspar 

 has been converted into a micaceous mineral the result is 

 an aggregate of minute twisted and crumpled, colourless 

 plates, which with crossed nicols polarise with much colour. 

 When these flakes are more regular and somewhat of larger 

 dimensions than usual I have been able to observe that 

 they narrowly resemble muscovite. As, however, both the 

 potash and the soda lime felspars are similarly altered, the 

 idea suggests itself that the resulting mineral may be either 

 a potash, a soda, or a lime mica, according to the constitution 

 of the felspar. This micaceous alteration may be observed 

 in every stage, from a small central aggregate of mica scales to 

 the complete conversion of the whole felspar crystal. 



4. Orthoclase. 



The monoclinic potash felspar is found in rocks in that 

 part of the area east of the Gum Forest where the granitic 

 rocks are the representatives of the intrusive masses. It is 

 also found in the quartzose diorites of Riley's Creek and 

 Swift's Creek as a subordinate constituent. 



It is rarely well crystallised in the quartz cliorite, but 

 generally occurs as irregularly bounded masses or imperfect 

 crystals. In some cases it forms a more or less essential 

 part, and thus causes such rocks to approach in character to 

 the granitites and amphibole granites of the fall to the Tambo 

 River in which it predominates. It is more subject to 

 alteration than the triclinic felspars (including microcline), 

 and the alterations are to kaolin as well as to mica. 



Inclusions are rare, and confined almost to apatite and to 

 a little magnetite. As the diorites become more basic in 

 constitution, orthoclase decreases in amount, until, as in the 

 anorthite diorites of Riley's Creek and in the amphi- 

 bole-gabbros of the Gum Forest, it is entirely absent. I have 

 found it always associated with quartz, with either horn- 

 blende or magnesia mica, or with both together, in company 

 with the felspars of groups 1 and 3. 



In addition to these rock-forming felspars, I have met 

 with one which is a constituent of a mineral vein at Power's 

 Reef, near Eureka. This reef is immediately in the con- 

 tact, and the vein mentioned passes through the quartz 

 diorites, and consists of two felspars, quartz, and broad plates 

 of dull-green magnesia mica. One felspar is pale flesh-red, 

 and has all the appearance of orthoclase. The other is 



