MICROSCOPIC STRUCTUEE OF SOME AUSTRALIAN ROCKS. 205 



hundredth of an inch long. There is no disposition on the part 

 of the felspars to form lines of flow. Most of the large felspars 

 are compounded, nearly all showing dark and light zones, with 

 crossed nicols. The olivines are micro-porphyritic, and all show 

 traces of decomposition. The secondary product is of an orange- 

 yellow colour, contrasting with the green serpentinous matter 

 that has a like origin in other basalts. One rather thick zeolite 

 on this slice shows a perfect series of coloured rings intersected 

 by the arms of a dark cross. 



Slice 18. — Basalt from Vegetable Creek, Emmaville, New 

 England, New South Wales. In ordinary light, with a low power 

 idiomorphic crystals of olivine, plagioclase and magnetite are seen 

 scattered through a finely granular ground-mass of augite and 

 felspar microlites. The olivines are fairly fresh although greatly 

 cracked and broken. The order of consolidation seems to have 

 been 1. magnetite, 2. olivine, 3. felspar, and 4. augite. 



Slice 19. — Basalt from Vegetable Creek, Emmaville, New 

 South Wales. This is another example of a basalt with a granular 

 augitic ground-mass. The magnetite occurs in grains and patches. 

 The olivines are idiomorphic and much corroded, and in some 

 instances contain cores of the ground-mass. There are two very 

 characteristic olivines nearly parallel to oo P ooon this slice showing 

 inclusions and corroded edges exceedingly well. Twins of olivine 

 are rare in most basalts I have met ; one example occurs in this 

 slice. 



Slices 20 and 21.— Basalt from Dubbo, New South Wales. This 

 basalt occurs in a great stream, running north and south, to the 

 east of Dubbo. Some patches about the township are probably 

 remnants of a later eruption. Macroscopically, the rock exhibits 

 a finely intimate crystalline structure. It is of a deep blue-black 

 colour, with a shade inclining to green. It is remarkable for the 

 quantity of steam vessicles that honeycomb the rock. These 

 cavities are often drawn out in one direction. Under the micro- 

 scope, the slice is seen stained a rich brown-red colour in patches. 

 Broad prismatic compound plagioclases are observed penetrating 



