110 Transactions of the Royal Society of Victoria. 



comparatively large irregularly-shaped masses, without any 

 striation, and obscuring in partial fields. As there is no 

 trace of cleavage, and as there are no bounding planes 

 developed, observations as to the angle of obscuration could 

 not be made. The percentage of potassa in the subjoined 

 analysis renders it, however, most likely that these felspars 

 are as I have classed them. Besides these larger individuals, 

 there are also smaller fragments of the same. The other 

 felspar is a plagioclase in much wasted crystals. The only 

 obscuration angles which I could measure with any confi- 

 dence, gave 2° approximately on OP (001), and 11° 

 on ooPoo (100.) The mode of twinning resembles that of 

 oligoclase. 



Muscovite mica is in large crystals which have been 

 much corroded at the sides. Where the section cats the 

 crystal at a slight angle with the base, the slice has a peculiar 

 mottled-appearance, due to the overlapping of numerous 

 consecutive cleavage plates. But where the section coincides 

 with the basal cleavage, the slice is optically perfectly 

 homogeneous, and polarizes with uniform tint of colour. 

 There are no inclusions, and the mica seems to me to be of 

 the same period of formation as the felspar, but to be 

 younger than the quartz. The quartz is in large masses, 

 filling in all spaces, and including portions of broken up 

 felspars of both kinds, and also small flakes of muscovite. I 

 made a quantitative analysis of a portion of this dyke 

 which was apparently but little decomposed, but also 

 somewhat more quartzose than the part examined under the 

 microscope : — 



Analysis 



No. 



5.- 



—Muscovite 



Granite. 



Si.02 









7610 



AI2O3 









15-95 



Fe.A 









tr. 



Ca.O 









•23 



Mg.O 









•11 



KoO 









3-27 



Na.20 









2^90 



H,0 



iopic 



... ... 



Moisture 



116 





99^72 



Hygrosc 



•18 



Sp. gr. 







... 



2-673 



