554 



Transactions. — Geology. 



ing amount of foreign matter into different portions of the 

 rock-mass, and the two samples may have come from different 

 parts of the same dyke or intrusion. 



Campbell Island is formed of volcanic and sedimentarv 

 rocks of perhaps early Tertiary age, resting on a platform of 

 metamorphic schists. The existence of granite dykes in these 

 is recorded in Filhol's " Mission de l'lle Campbell," page 143, 

 but no diorite dykes are mentioned. Such intrusive rocks 

 might be expected, to occur in the off islands of New Zealand, 

 as they are not truly oceanic, but part of a continental area 

 which stretched chiefly to the east and south, and which was 

 above sea-level, either wholly or partly, during late Tertiary 

 times. 



No. 2. 



This is a basic glass or tachylyte — specific gravity about 

 2'5, hardness 4-5, with a brown streak, fusibility about 3 ; 

 the water-percentage I found to be 6-3. But the following 

 analysis, taken from the report of the Colonial Laboratory for 

 the year 1903, gives details of its chemical composition : — 



541 



1000 



This analysis shows the usual characteristic of a basic 

 gl ass — viz., the high percentage of silica ; but it is probable 

 that it may be the glassy equivalent of the andesites found on 

 the island. 



Microscopic examination shows that it is undoubtedly a 

 glass, rendered almost opaque by magnetite-dust. This is 

 aggregated at times into cumulous masses, which sometimes 

 show radial arrangement, the intervening spaces being more 

 translucent. Perlitic cracks, due to cooling, are common. 

 Porphyritic crystals of plagioclase and augite of very small 

 dimensions also occur very sparingly. 



When placed near the pole of an electro-magnet almost 

 all the powdered rock was strongly attracted, due, no doubt, 

 to the excessive amount of magnetite-dust disseminated 

 throughout it. 



