Soiitlt Australian Dolomites. 493 



medium and of a granular nature. The specific gravity is 3"25. In 

 many cases an incipient alteration to a pleochroic actinolite can be 

 observed in the slides. This may develop peripherally or in isolated 

 spots through the grain. In the associated quartz veinlets flakes of 

 graphite and little rounded grains of diopside are often present. 

 Graphite also occurs along j oint faces. The diopside rock occurring as 

 an inclusion in the igneous gneisses of Fishery Bay is essentially 

 similar, but here it has been invaded by the magma in the form of 

 felspar veins. 



In these cases the felspar has been very prone to alteration, and 

 in some slides it is represented by its decomposition products as 

 strings through the diopside grains or between and at the corners of 

 these individuals. The original felspar was a plagioclase and had 

 a refractive index greater than C. balsam. In some cases it was as 

 basic as a calcic andesine. Scapolite and zoisite are present in 

 addition to sericite as decomposition products. Hornblende may in 

 some cases be seen as streaks traversing the rock in hand-specimens. 

 Under the microscopie an examjDle of this type was seen to consist 

 of diopside and hornblende intergrown without any clear evidence 

 that the amphibole has been derived from it. 



A slice cut from a diopside rock developed as a result of the 

 intrusion of a quartz vein in the dolomite of the Waterfall Creek, 

 Hundred of Hutchison, shows greyish diopside, tremolite, and quartz. 

 The rock has been crushed and the tremolite is the result of the 

 crushing of the diopside. The quartz shows undulose extinction, 

 and is in contact with tremolite, this relation being contrary to that 

 expected, if tremolite had arisen as a direct result of silication. 



(6) Dio-pside-Microcline Rocks. 



East of the diopside headland the rocks pass into more aluminous 

 beds, in which diopside is still an important constituent. These 

 rocks have all the appearances of representing a transition into 

 calcareous shales and sandstones from purer dolomitic sediments 

 prior to metamorphism. They are interbedded with rocks that are 

 now garnet gneisses, sometimes carrying sillimanite, and these are 

 probably also metamorphosed sediments. 



The diopside-bearing rocks have also suffered intrusion from 

 quartz solutions, as represented by quartz veinlets, and strings 

 traversing them, and these on weathering give the rock quite a 

 serrated appearance. When examined under the microscope these 

 rocks are seen to contain the following silicates, to the exclusion of 

 all carbonate minerals — diopside, microcline, plagioclase, scapolite, 

 biotite, titanite, zoisite, and quartz. Graphite is also usually 

 present in small amount. A typical section is described as under. 



No. 160. Diopside is present in light-green coloured grains and 

 often shows crystal outlines. The extinction angles of these 

 diopsides reach a value of 44 degrees, and the light-green colour of 

 the mineral suggests the presence of the hedenbergite molecule 



