626 Transactions. — Geology. 



seen with the naked eye. The feldspar is probably iabradorite 

 or anortbite, as the method of determination by the extinction 

 of twin lamellae gave very high angles. Some of the crystals 

 exhibit zonal structure. The crystals of augite are small 

 and occasionally twinned. The ground-mass consists almost 

 wholly of glass, in which are feldspar microliths and grains of 

 magnetite. The rock must therefore be classified as a volcanic 

 glass, and the presence of magnetite and augite shows that it 

 belongs to the basic series. From its chemical and physical 

 properties it would probably be called a tachylyte. This rock 

 appears to be the same as that described by Professor Thomas, 

 but I did not notice the pleochroic mineral mentioned by him. 



Andesite (Macaulay Island). 



This rock is dark in appearance, and has a specific gravity 

 of 2-87. Small crystals of porphyritic feldspar are visible to 

 the naked eye. On examining it microscopically this appears 

 to be the only porphyritic mineral, though the specimen was 

 so small that others might very well exist. The angles of 

 extinction were again high, so that it is probably Iabradorite 

 or anortbite. The feldspar is not weathered, and contains 

 numerous sirall inclusions, and often shows undulose extinc- 

 tion. The ground-mass is semicrystalline, and contains mi- 

 croliths of feldspar and other dark material. The rock ap- 

 pears to belong to the andesite group, both from its specific 

 gravity and its microscopic appearance ; but the absence of 

 any ferro-magnesian mineral renders its accurate classifica- 

 tion difficult. 



Augite-andesite (Macaulay Island). 



The external appearance of this rock is dark-grey, with 

 feldspar crystals plainly visible. The specific gravity is 2-7. 

 On examining it microscopically it appears to be composed of 

 a semicrystalline ground-mass, with porphyritic crystals of 

 feldspar and augite. The feldspar is in moderately large 

 crystals up to ^iu. in length. They are clear and free from 

 inclusions. The extinction-angles render it probable that it 

 is Iabradorite or anortbite. The crystals of augite are small. 

 The ground-mass is full of grains of magnetite and feldspar 

 microliths ; very little glass is present. The rock therefore 

 appears to be an augite-andesite. 



Andesite (Sunday Island). 



This specimen was much weathered, and an accurate 

 description is therefore difficult. It is light-grey in colour. 

 Specific gravity, 2-55. Crystals of feldspar are visible with the 

 naked eye, and under the microscope show the twinning of 

 plagioclase. From the extinction-angles it is probably lab- 



