196 REV. J. MILNE CUKRAN. 



clase, augite, olivine and magnetite. Some of the large olivines 

 show an interesting example of the process of alteration of olivine 

 to serpentine. The green serpentine has filled up cracks and 

 eaten its way a little on either side. The serpentine is then seen 

 sending out spear-shaped points into the undecomposed substance. 



Slice 2. — Basalt, Star Lead, Gulgong. This basalt varies from 

 a pilotaxitic to a hyalopilitic, but the glassy matter is neither 

 commonly seen nor great in quantity. The rock is wholly crystal- 

 line, there being present no interstitial matter. Under the 

 microscope, the rock is remarkable in structure for the rather 

 peculiar way in which the magnetite is wedged in between the 

 other minerals. This rock differs from most basalts in the absence 

 of anything approaching porphyritic crystals of olivine or augite. 

 The augite is, for the most part, in grains also wedged in between 

 the felspars. A great deal of the magnetite appears to be of 

 secondary origin, as it shapes itself around the plagioclase crystals, 

 while on the contrary, some true cubes of this mineral are found 

 included in the felspars. As in the rock, just described, magne- 

 tite sometimes runs in irregular strings to a considerable distance 

 in the rock. The average length of the lath-shaped felspars may 

 be put down as the one-hundredth part of an inch. The small 

 patches of glass are rendered almost opaque by the presence of 

 granular magnetite. There is no streaming of the felspars noticed 

 around large crystals, but the majority of the felspars point in 

 one direction which may be taken as evidence of a flow structure. 



Slice 3. — Tertiary Basalt, Star Lead, Gulgong, New South 

 Wales. Macroscopically this rock is of a bluish-grey colour. It 

 has a general appearance of having undergone considerable alter- 

 ation without approaching to anything that might be called 

 decomposition. There is nothing sparkling that catches the eye, 

 as in basalts generally, and when struck with the hammer the 

 ring is rather dead. Aggregations rather than porphyritic 

 crystals of a dark green mineral are barely recognisable. Under 

 the microscope in plane light, the peculiar disposition of the 

 magnetite is the first thing to attract one's notice. This mineral 



