Western Australia. 113 



Several facts, however, have liitherto completely escaped notice. 

 In the first place, while the rocks in general are undoubtedly composed 

 almost entirely of felspar, and the mineral occurs frequently as 

 phenocrysts, there are quite a number of instances in which no 

 phenocrysts are present, and the rock is apparently wholly made up of 

 small rectangular and columnar latlis of the mineral, without, however, 

 any ' flow ' or ' trachytic ' structure. Moreover, the character of the 

 felspar is peculiar. Both measurements of the extinction angles on 

 albite lamellae and chemical analysis show that the mineral is 

 particularly rich in soda; indeed, the percentage of lime in the 

 analysis is so small that it can no doubt be all accounted for by the 

 presence of (decomposed) ferro-magnesians. No potash whatever 

 appears in the analysis. 



Further, there are indisputable evidences of the former presence 

 of phenocrysts of hornblende : six-sided crystal forms sliowing 

 prismatic angles which measured 125° and 55° have been observed 

 in several instances, and though, owing to chloritization of the 

 mineral, the usual prismatic cleavages are not observable in cross- 

 sections, the six-sided form, so common in cross-sections of horn- 

 blende, and to a less extent the occurrence of rectangular and 

 rliombic longitudinal sections, leave no doubt as to the identity of the 

 mineral. 



As will shortly be shown, a comparison of the composition, 

 characters, and structure of the rocks establishes their identity with 

 that group which Rosenbusch calls quartz-keratophyres, but which 

 also appears elsewhere as albite-porphyrites. In many ways they 

 agree also with varieties of quartz-andesite, but, though it was 

 impossible to prove the presence of fluid cavities in the quartz, 

 there is no indication of any glassy base ; the felspar is evidently 

 albite, and, according to the field evidence, as has been stated, the 

 rocks are at times distinctly intrusive as tongues and narrow dykes. 



(a) The Porphyritic 7>j9e.— 12331a, 12331c. G.M.L., 4406 E., 

 Hyman. 



M.C.^ A grey rock with faint reddish tinge, and with one 

 enclosure of large size and several of small size, all containing 

 strings or small nests of tourmaline needles. 



S.' Minerals observed: felspar, quartz, chloritized hornblende, 

 niuscovite, browrf and black iron-ore in crystals and grains, chlorite. 



In ordinary light the section shows scales and flakes of green 

 pleochroic chlorite, brown and black grains of iron-ore, and various 

 sections of chloritized phenocrysts in a clear or slightly brown - 

 stained ground (PI. VI, Fig. 1). The chloritic sections occur in 

 rhombic, rectangular, and clearly six-sided shapes, and without 

 any doubt represent original hornblende. No tourmaline was seen in 

 the particular section examined, though its presence was visible to 

 the eye in isolated spots in the rock. 



Under crossed nicols, besides the chloritized phenocrysts, appear 



large idiomorphs of fresh, often minutely twinned felspar in 



a ground-mass of minute laths of the same mineral, and small plates 



and platy aggregates of quartz (PI. VI, Fig. 2). Measurements show 



^ M.C. = macroscopic characters; S. = section. 



DECADE VI. — VOL. I. — NO. III. 8 



