BY H. C. TilCHARDS. 191 



microscope {see Plate xii, figs. I, 2) the section shows a 

 holocnstalline texture although the groundmass is very- 

 fine. Phenocrvsts of a.uorthoclase, sanidine and nosean 

 occur. The sanidine crystals range up to 0.5 mm. in length 

 and are idiomorphic, whereas those of anorthoclase are 

 allotriomorphic and up to 1 mm. long, while the nosean 

 crystals are allotriomorphic to hypidiomorphic and the 

 rounded granules range up to 0.25 mm. in diameter. The 

 fluxion structure can be seen in a general way in the section 

 in the ground mass which is composed of felspar, nosean, 

 very fine light-brown augite granules and magnetite. 

 The nosean is rather abundant and the granules are trans- 

 parent and isotropic with rough cracks through them. 

 The refractive index is much lower than that of Canada 

 balsam. When altered, the granules become reddish -brown 

 and this brown staining can be seen developing along the 

 cracks. The nosean was determined by etching A\ith strong 

 hydrochloric acid for 30 minutes and then staining the 

 gelatinous silica produced thereby with fuchsine. Some of 

 the granules showed a weak double refraction but the 

 dust like inclusions, so generally characteristic of nosean, 

 Avere absent. 



The alteration into brown limonite and a kaolin-like 

 aggregation is, according to Iddings*, one of the characteristic 

 weatherings of nosean and this, together Avith the very low 

 lime content of the rock, inclines one to nosean rather than 

 hau3-ne. The analysis of this rock, as shown on Table I, 

 shows the very alkaline character, with 10. Gl per cent, of 

 alkalies and less than one per cent, of lime. Although the 

 silica percentage is 67.32 the rock is regarded as a phono- 

 lite The sjDecific gravity is 2.519. 



Specimen 283, is from the Opal Ridge and is also 

 characteristic of the material on Gill's Hill and on Mt. 

 Boorambool. The material is somewhat weathered and 

 veins of chalcedony along the fluxion lines and through 

 the cracks are prevalent. The rock is very fine -grained 

 and in colour is pinkish-brown. It is reported that this 

 is the type of material in which the precious opal was found. 

 Under the microscope the texture is hypohyaline and the 



*Rock Minerals, 1911 Edtn, p. 257. 



