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SUBANTARCTIC ISLANDS OF NEW ZEALAND, [Physiography, Geology. 



The analysis shows the rock to be of a basic type, with a remarkably high per- 

 centage of iron-oxides. This is confirmed by the microscope, as the groundmass 

 contains an abnormal amount of titaniferous magnetite. There are phenocrysts 

 of andesine-labradorite, and occasional small phenocrysts of augite, grains of 

 which mineral also occur in the groundmass. This rock is probably an old 

 lava-flow. 



The special petrological characters of the diabases appear to be the presence 

 of purple pleochroic augite, often in large ophitic plates, and a remarkable amount 

 of titaniferous magnetite. Epidote is a common constituent of some of the rocks, 

 and secondary hornblende and mica are sparingly found. In many cases there is 

 much chlorite present, probably derived from the augite ; apatite needles and occa- 

 sional grains of quartz also occur, and in some of the later flows there is much 

 secondary calcite. Besides the true diabases, diabase-porphyrites also occur, and 

 these grade into feldspar-porphyrites of more acid type. The age of these rocks 

 cannot be exactly determined, as they are associated with no sedimentaries, but 

 from their petrological character it is almost certain that they are older than early 

 Tertiary — how much it is impossible to say. 



The dykes penetrating these rocks are of varied character ; there are dolerites 

 with much olivine in large crystals (no doubt belonging to the later basic series), 

 porphyrites, and diabases (fig. 14, iii), and a large development of a coarse-grained 

 porphyritic olivine-diabase. This rock contains large feldspars (labradorite), up 

 to 1 cm. in length, and occasional augite phenocrysts in a coarse holocrystalline 

 groundmass composed of feldspar laths, altered augite, and olivine. The ground- 

 mass also contains a considerable quantity of chlorite derived from the P.M. 

 minerals, and much titaniferous magnetite in grains and comb-shaped forms ; a good 

 deal of the latter is no doubt of secondary origin. Secondary carbonates occur, 

 and apatite needles are occasionally seen. The following analysis, by Mr. A. M. 

 Wright, shows its composition, and accords with the microscopical characters : — 



Per Cent. 



Si02 













. 49-16 



A1203 













. 15-27 



Fe^Og 













8-64 



FeO 













7-22 



MnO 













0-29 



CaO 













5-26 



MgO 













8-34 



K2O 













0-61 



NagO 













2-72 



P2O5 













Trace 



Ti02 













0-48 



CO, 













0-71 



Water lost below 10( 



J°C. (H2O-) . 







0-44 



„ above 10( 



)°C. (H^O-f) . 







0-98 





100-12 



Specific gra'^ 



ritj 



, 



, , 



, , 



, , 



2-70 



