113 



making their recognition easy in polarized or ordinary light. 

 These intergrowths so closely resemble the figures and de- 

 scriptions given by Rosenbusch of the microline-albite in- 

 tergrowths that the narrow bright strips are referred to that 

 mineral, though it was impossible to obtain determinative 

 readings of their extinction angles. Such readings were ren- 

 dered uncertain by the narrowness of the lamellae ; the 

 angles, such as were obtainable, were generally small, though 

 one reading gave 18°. 



On the whole, the microcline is very free from kaolini- 

 zation ; dusting with epidote and rutile ( ?) is, however, not 

 unusual . 



Microcline occurs in a variety of forms. In the gneissic 

 aplite near the school-house it is present in large amount as 

 irregular grains, and with much albite intergrowth. It is 

 the predominant mineral in the rock that occurs to about a 

 mile south of Inglewood, but becomes merely an accessory 

 mineral in rocks further to the north. In these it is usually 

 present interstitially, though occasionally forming large 

 squarish but anhedral crystals. 



Orthoclase is not so common as either of the preceding 

 felspars, whilst nearly every rock contains some potash fel- 

 spar. It is most usually in the form of microcline, particu- 

 larly where the rocks give most sign of having undergone 

 heavy strain. It is a well -recognized fact that orthoclase may 

 assume the microcline structure under strong pressure. <-'5^ 

 The presence in these rocks of strained orthoclase assuming 

 a moire appearance, and finally taking on a true "gitter 

 mikroklin," is not an unusual feature. 



Orthoclase forms in white anhedrons varying in size up 

 to 2 mm. in diameter. Where still unaltered to microcline, 

 as in parts of the gneissic aplite, it may be full of micro- 

 perthitically intergrown stripes of albite. In a granite that 

 occurs in the Torrens Gorge just west of the main intrusion, 

 but certainly a product of it, orthoclase is almost as abundant 

 as plagioclase. In the syenite near Mr. Scrympngers it is 

 the predominant mineral. In the gorge and to the east of 

 the intrusion is a monzonitic rock, in which orthoclase is pre- 

 sent also in large amount, Carlsbad twinned and somewhat 

 kaolinized. In the rock occurring in the Houghton Cemetery 

 Reserve it is present in small amount, forming in small and 

 fairly fresh grains, while it occurs in large untwinned grains 

 in the rocks of the eastern vein on the Gumeracha Road. 



The alterations imdergone by orthoclase are quite normal. 



(13) Rosenbusch, Microscopical Physiography of Rock-formiiig 

 Minerals, Idding's Trans., Fourth Edition, p. 32€. 



