064 REPORT— 1901. 



Point diorites, in wlaicli hornblende is notably present, with tbe result stated 

 in the accompanying petrographic note. It is intended to continue the systematic 

 investigation of all the Victorian so-called diorites, particularly those with which 

 auriferous quartz veins are associated. 



In this investigation I shall have the valuable co-operation of Mr. F. P, 

 Menuell, an Australian student at the Royal School of Mines, London. 



The following brief description is intended as a preliminary note : — 



Wood's Point, "Victoria. 



Slide 277. — This slice was cut from a dark coloured, even-grained rock of 

 granitic aspect. The specific gravity is high (2'9). Black hornblende is the most 

 conspicuous constituent; ilmenite and pyrites can also be recognised by their 

 characteristic colour and lustre. Under the microscope the rock does not show 

 that simplicity of structure which might be inferred from its appearance in hand 

 specimens. Hornblende is still the mineral which gives a distinctive character to 

 the rock ; but the whitish material with which it is associated, though much 

 decomposed, is at once seen to be of a complex nature. 



Constituent Minerals: Hornblende. — This mineral occurs in large granules, 

 often showing crystal faces, though the outline is frequently too indefinite for the 

 form to be determined with precision. The prismatic cleavage is generally well 

 marked, though some crystals show irregular cracks. The colour is in mort cases 

 brown, though some of the crystals are of a greenish tinge, and a few are quite 

 colourless. The coloured varieties exhibit strong pleochroism (fairly deep brown 

 to almost colourless). Sections showing only one set of cleavage traces give a 

 maximum extinction angle of 20°. 



Felspar. — The predominant felspar is evidently plagioclase, though owing to 

 its decomposed state and the absence of twin lamellation or cleavage traces it is 

 difficult to assign it with certainty to its proper position in the albite-anorthite 

 series. It seems, however, to be a basic oligoclase, and it is notable that in one 

 or two instances it presents crystal faces to the hornblende. Orthoclase is also 

 present, chiefly intergrown in crystallographic relation with quartz, forming 

 micropegmatitic patches, which give to portions of the rock very much the 

 appearance of a granophyre. 



Quartz occurs almost entirely in micrographic intergrowth with the orthoclase 

 as sliarply defined skeleton crystals, often triangular in outline. It is thus of 

 prior consolidation to the felspar with which it is associated, and in thin section 

 is the more distinct from its being entirely unaflected by the agencies which have 

 rendered the felspar almost opaque. 



Ilmenite is abundant in irregular grains and skeleton crystals, and is, no 

 doubt, the source of the black ' titaniferous ironsand ' which is so plentiful in the 

 locality. Its outline, lustre, and characteristic alteration atibrd a ready means of 

 identification. 



Sphene, of the white variety known as leucoxene, has been abundantly pro- 

 duced by the decomposition of the ilmenite. It does not form definite crystals, 

 but it serves to bring out the internal structure of the ilmenite in a most striking 

 manner, owing to the way in which decomposition has proceeded along the lines 

 of least resistance, related to the crystalline form (hexagonal) of the original 

 mineral. 



Other accessories are pyrites and apatite, neither of which is plentiful. The 

 former is easily recognised by its pale brassy colour, as seen by reflected light. 

 The apatite forms slender prisms, longitudinal sections showing the cross-fracture, 

 while transverse ones show the characteristic six-sided form. A colourless 

 mineral with the roughened appearance characteristic of a high refractive inde.Y 

 also occurs as a decomposition product of the hornblende. It is almost isotropic, 

 and may be referred to the chlorite group. 



Structure. — The texture and structure vary considerably in different parts of 

 the slice. The rock is holocrystalline, but the order of crystallisation of the 

 different minerals is variable and the presence of micropegmatite is distinctive. 

 The other minerals act very much the part of a ground mass toward the horn- 



