290 C. A. SUSSMILCH AND T. W. E. DAVID. 



Toscanites. — The rocks outcropping at the top of Mount 

 Johnstone, and known by the convenient field designation 

 of Paterson rhyolite, really belong to the type known as 

 Toscanite, characterised by a fairly basic plagioclase, and 

 in addition a notable though usually subordinate proportion 

 of orthoclase. The chief minerals visible microscopically 

 in the Mount Johnstone rock are, in order of abundance — 

 plagioclase (zoned basic andesine), quartz, orthoclase and 

 biotite. The ground mass has flow-structure, and is devitri- 

 fied to some extent, with the production of some spherulitic 

 structure. With the toscanites may also be included a 

 rock from the top of Mount Gilmore, which is practically 

 identical with the Mount Johnstone type. 



Dacites. — By the decrease and virtual elimination of 

 orthoclase the toscanites pass over into dacites. These 

 are always porphyritic in quartz and felspar and generally 

 show ferro-magnesian minerals as well. Under the micro- 

 scope they are seen to be composed of plagioclase (andesine 

 or acid labradorite), quartz, ilmenite, biotite and hornblende 

 set in a groundmass which may be hypocrystalline or crypto- 

 crystalline and is sometimes partly spherulitic. Ortho- 

 clase is an occasional porphyritic constituent, as in one of 

 the flows at the top of the Eelah section. Generally both 

 biotite and hornblende are present, the latter often being 

 the more important. Quartz may sometimes be very sub- 

 ordinate among the phenocrysts. 



What is probably a pyroxene dacite occurs about the 

 middle of the Eelah section. The pyroxene, if such it be, 

 is now represented only by chloritic pseudomorphs. 



There is a group of dacitic rocks coming into the Mount 

 Johnstone section which have distinctive characters. The 

 specimens examined were collected east of the main road 

 at the foot of Mount Johnstone. They are all distinguished 

 by a very stony or felsitic appearance, the body colour 



