ROCK SPECIMENS PROM CENTRAL AND WESTERN AUSTRALIA. 



305 



typically opbitic forms. 

 In some cases it veins 

 the augite in a peculiar 

 manner that suggests 

 coarse intergrowths 

 (Fig. 2) and at times is 

 clearly intergrown with 

 the enstatite-augite on 

 a fine scale (Fig. 5, Plate 

 XIV). 



The monoclinic pyrox- 



Figure 2.—Polysomatic group of enes, though all optically 

 enstatite-augite with intergrowth of positive, vary consider- 

 hypersthene Dolerite Cavanagh aM [r tfeal { 



Manqe. Magnification 15 aiams. 



from practically 90 to 



0*4°. It has been shown by Wahl 1 that this variation is 

 dependent on the amount of lime entering into the compo- 

 sition of the mineral. In normal g!iopside, with high axial 

 angles, the proportion of OaO : MgO + FeO is nearly 1 : 1, 

 but in a series of pyroxenes in which the proportion of lime 

 is gradually less, the axial angle is correspondingly smaller, 

 until it passes through 0° and the optic axes open out in a 

 plane normal to the plane of symmetry, so that in certain 

 cases the mineral is uniaxial for a given colour. For this 

 s eries, which he supposes to consist of solid solutions of 

 ordinary diopside or augite on the one hand, and clino- 

 enstatite or clino-hypersthene on the other, Wahl has 

 proposed the generic name of enstatite-augite, with specific 

 names for different members of the series. Rosenbusch 2 

 has preferred to call the group magnesian-diopsides. These 

 peculiar augites have been recognised in Australia so far 



1 Wahl, W\, Die Enstatitaugite. Tsch. mia. u. petr. Mitth. xxvr, 

 (1907), pp. 1 - 31. 



2 Rosenbusch, H„ Mikr. Phys., i, 2. 



T— Nov. 1, 1911. 



