156 R. A. Farquharson — Petrology of N. Kalgoorlie, 



Minerals observed : quartz, felspar, ])Trite, magnetite, granular 

 calcite, chlorite, a micaceous mineral (muscovite). 



There are long broad columnar crystals of a felspar showing either 

 only Carlsbad twinning or none at all ; other crystals, tabular as 

 well as columnar, show broad albite lamellation and symmetrical 

 extinction angles as high as 14°, 15°. The former are doubtless 

 orthoclase, the latter andesine or a basic oligoclase. Quartz is 

 abundant in a few plates that are almost idiomorphic, but mostly in 

 shapeless areas. Intergrowths of felspar and quartz are very frequent, 

 and of large extent, the felspar being either micacized or kaolinized. 

 In some places tabular felspars appear like islands in a sea of clear 

 quartz ; in others the felspar of an intergrowth is optically continuous 

 Avith a tabular crystal. Instances are to be seen where felspar crystals 

 have been broken across, and in these the cracks have all become 

 filled with clear quartz. 



Granular patches of calcite and magnetite and also of chlorite, with 

 crystal outlines, may represent an original ferro-magnesian. Flakes 

 of a colourless micaceous mineral occur sporadically and have been 

 referred to muscovite. Pyrites is common in the rock in grains of 

 various size. There is no distinct porphyritic structure. 



The rock is difficult to name ; it resembles some hornblende granites 

 and quartz diorites, but, though no ground- mass is noticeable, it more 

 closely approaches in character a granite porphyry. The great amount 

 of clear quartz and tlie infilling of cracks in the felspars by this 

 mineral suggests a partial metasomatic replacement of some of the 

 felspar by quartz. 



2918 is a grey rock with dark-grey spots in a lighter rock mass, 

 and with veins of pyrites. In section are visible large shapeless 

 plates of clear quartz, some leucoxene areas, and much granular 

 pyrites in a granular mass of carbonate, quartz, muscovite, and 

 sericite. There are remains of platy felspars now sericitized, and 

 occasionally what appear to have been intergrowths of quartz 

 and felspar, the latter sericitized. 



No noticeable rock structure now remains, so that it is impossible 

 to come to any conclusion as to the nature of the original rock. 



5. Conclusions. 

 Briefl.y, the main points which emerge from a consideration of the 

 area dealt with in this paper are these. The earliest rocks and those 

 that form the greater portion of the area are greenstones which 

 comprise masses of doleritic or gabbroid type, otliers of amphibolite 

 without doubt derived by extreme dynamic metamorphic action from 

 the former, and talc-chlorite rocks which probably represent the 

 extreme phase of chemical and dynamic alteration both of the dolerite 

 and of the amphibolite. The movements which contributed to the 

 pi'oduction of the hornblende of the amphibolite from the augite of 

 the dolerites or gabbros not only produced a shearing in the rocks 

 themselveSj but developed actual fissures in the rock masses, a main 

 series at right angles to the direction of stress and no doubt a minor 

 at riglit angles to the latter. Moreover, along several lines the 

 shearing and crushing stresses were probably greater than along 



