Notes on Monvliiqiiile Dijkes. 5 



Vi'tnxjnt ph icdl ( 'Jiaracters. 



Central Red. White and Blue Hork. :n8-f(K>t level. (Plate T, Fig. 1). 

 This rock in the hand s))ecinien is dense, bluish-black, and very hard. 

 Porphyritic crystals of olivine can be seen, and also occasional white 

 vesicles tilled with calcite. 



Microscopically, it is extremely dense. Occasional very large olivine 

 crystals are found which are more or less completely serpentinised. 

 Smaller olivine plienocrysts are common, and are for the most part 

 relatively clear and unaltered. These fresh olivines from the 318-foot 

 level are remarkable when one considers that the olivine usually seen 

 in the Bendigo rock sections is very much altered, even at the deepest 

 levels of the Victoria Quartz Mine, 4(514 feet. 



Augite is perhaps the most abundant mineral. It is a ])urii]e, titaiii- 

 ferous variety, and faintly pleochroic. A brown prismatic hornblende 

 is present in a- much smaller amount than augite. It is strongly pleo- 

 chroic, and is distinguished from biotite mainly by its oblique extinc- 

 tion. The angle between the prism faces can only sometimes be seen. 



Ilmenite is extremely abundant in small crystals distributed evenly 

 throughout the rock. Microlites of ilmenite are also abundant. Tliin, 

 colourless rods of apatite are discernible in the ground mass. 



The ground mass when unaltered is clear, colourless and isotropic. 

 It-5 X'efractive index is higher than that of xylol (1.4912) and not much 

 different from that of a sample of cedar oil (1.5090'). The ground 

 uuiss is therefore a glassy residuum, and not analcite. Not only does 

 the material serve as a general ground mass for the whole of the rock, 

 but it appears as well here and there in irregularly-shaped areas which, 

 tontaining an excess of brown hornblende, form light-coloured patches 

 ill the rock. These are obviously the acid residuum remaining after 

 the crystallisation of the bulk of the magma. In part the ground 

 uuiss is found to be not absolutely isotropic and to show polarisation 

 colours up to iron grey and grey white. This condition is most evident 

 in some of the segregated patches when in addition a curious perthitic 

 intergrowth with ilmenite is evident. It is here found to pass out into 

 occasional basic felspar laths, and the assumption is that the 

 remainder of the crystallised ground mass is felspar. 



Vesicles in this rock are tilled mostly with a carbonate, probably 

 calcite. The carbonate could well be dolomite, especially as rhombs of 

 til is carbonate have been seen in sections from the rocks from other 

 mines. Occasionally rods of a fibrous carbonate, arragonite or fibrous 

 calcite, can be seen under the high power lining the edge of some of 

 the vesicles. Rods of a fibrous zeolite, probably natrolite, are present. 

 Analcite, in snudl isotropic cubes, has also been seen. Chalcedonic 

 silica is present as a secondary mineral, and also crystallised silica as 



