Physiography, Geology.] SUBANTARCTIC ISLANDS OF NEW ZEALAND. 733 



Microscopic. — The groundmass is composed chiefly of granular augite, olivine, 

 and magnetite, with a lesser proportion of feldspar laths. Phenocrysts are of very 

 coarse slightly pleochroic augite, occasionally twinned, and coarse round olivine 

 grains, with a border and seams of serpentine and limonite. This rock is similar 

 to that found capping the hills in other parts of the group, and from it the large 

 augites have weathered which are found occasionally on the surface in considerable 

 quantity. 



The following are analyses made by Mr. A. M. 

 shore at Adams Island ; No. 4, a dolerite from a 

 Island : — 



SiOo 



Al.O, 



Fe,0, 



FeO 



MnO 



TiOo 



CaO^ 



MgO 



K2O 



NaaO 



P2O5 



CO, 



Water lost below 100° C. (H2O-) 

 above 100° C. (H2O + ) 



Wright- 



No. 1, a basa 



It from 



height of 



1,700 ft. at 



Adams 



No. 1. 



No. 4. 





Per Cent. 



Per Cent. 





46-18 



47-14 





16-82 



19-35 





2-08 



5-61 





13-46 



10-82 





0-27 



0-18 





0-51 



0-68 





4-81 



5-42 





5-68 



3-98 





1-83 



1-60 





5-02 



2-38 





1-05 



0-14 





0-31 



0-26 





0-89 



0-84 





0-82 



0-96 





99-73 



99-36 





Specific gravity . . . . . . 2-88 2-96 



These two analyses were made from rocks belonging to the fine type and the 

 coarse type. They show no marked difference in basicity, but a high per- 

 centage of ferrous iron ; the percentage of magnesia is somewhat low. In No. 1 we 

 have a very high percentage of soda, suggesting the presence of a felspathoid, but 

 no sign of this could be observed. 



There seems to be no well-defined order in the character of the lavas extruded, 

 but merely an alternation of coarse- and fine-grained types right from the earliest 

 rocks erupted. 



A similar series of specimens, taken at the head of the North Arm, was prac- 

 tically basaltic throughout, with the exception of a basic andesite occurring near the 

 top of the series. The series shows the same alternation of coarse- and fine-grained 

 rocks. 



The following features of the minerals of the basaltic series may be noted : The 

 augite is notably titaniferous, as shown by the frequent pleochroism and dark-violet 

 border. Ilmenite or titaniferous magnetite is of very common occurrence not only 

 in the basalts, but in the diabases and diabase-prophyrites which occur as dykes 

 penetrating the basalts. The feldspars are not of very basic character, and vary 



