Vol. 69.] PETROLOGY OP THE KALOOOKLIE GOLDPIELD. 647 



described, and dense rocks hardly to be distinguished from the 

 older fine-grained greenstones. Both groups, again, occur in 

 perfectly massive and perfectly schistose forms ; but the green 

 chlorite-schists are quite subordinate in amount to the pale 

 quartz-sericite-carbonate schists. 



A constant characteristic in all but the most schistose members 

 is the presence of dull yellowish to purple patches of leucoxene, 

 ■which are most clearly evident on wetted surfaces. Even where 

 there is a considerable development of magnetite or pyrite, part, 

 at least, of the leucoxene is commonly retained. In some of the 

 massive greenstones the outlines of the felspars are visible in hand- 

 specimens, and their prismatic forms have led more than one 

 observer to describe them as phenocrysts. 1 This deceptive appearance 

 is due to the absence of distinct pseudomorphs of the original 

 ferromagnesian minerals, which have altered to irregular masses 

 of chlorite and carbonates and may easily be confused with the 

 altered ground-mass of a porphyritic rock. Black blebs of quartz 

 are abundant in the coarser ro«ks, the black being due merely 

 to the background of chlorite in which the quartz is set. 



The most strongly chloritic rocks are found in the ' North End ' 

 and on the western margin of ' The Mile' : that is, in the proximity 

 of the amphibolit.es. In section, the distinctive elements of the 

 quartz-dolerites can clearly be recognized. The felspars are often 

 well preserved as to form, although much coloured and obscured 

 by chlorite along the cracks and cleavage-planes. They form large 

 tabular crystals, often twinned on the albite law, less often on the 

 pericline, and occasionally on rarer laws ; but Carlsbad twins are 

 seldom, if ever, seen. Their identification as albite rests on the 

 following observations: — maximum extinctions on symmetrically- 

 cut albite-twin lamellae of 16°, refringence lower than that of balsam, 

 maximum birefringence always low, optical character positive. 

 On the felspars are moulded highly-embayed crystals of ilmenite, 

 more or less altered to leucoxene. At times, there is in addition a 

 considerable amount of octahedral magnetite, which in some cases 

 encloses the ilmenite or leucoxene. A former mineral, which 

 embraced the felspars ophitically, is represented by aggregates 

 consisting principally of chlorite, with smaller amounts of carbonates 

 and in a few cases of epidote. These aggregates are assumed to be 

 the alteration-products of the original augite of quartz-dolerites, 

 but there is no intrinsic evidence to prove that such is the case. 

 The interstices between the above-described minerals (or mineral 

 aggregates) are occupied by coarse or fine micropegmatite, the 

 felspar of which is often polysynthetically twinned and may some- 

 times be seen to be continuous with that of the large columnar 

 crystals. Apatite occurs as inclusions in all the above minerals, 

 but is most abundant by far in the micropegmatite as long slender 

 needles. Tourmaline is a rare accessory. 



In rocks such as those described above, where the original 



1 As previously stated, Prof. Judd and Mr. Larconibe have termed these 

 rocks ' quartz-andesites.' 



2u2 



