736 



SUBANTARCTIC ISLANDS OF NEW ZEALAND. [Physiogra'phy, Geology. 



as well in the groundmass, but they have very few idiomorphic augite phenocrysts, 

 the phenocrysts being an occasional andesine-labradorite feldspar. The following 

 analyses, by Mr. A. M. Wright, are typical of these rocks (A is a basic dyke 

 from Adams Island, and B a basic porphyrite dyke in gabbro from Carnley 

 Harbour) : — 



SiOg 



AIO3 



FeOg 



FeO 



MnO 



TiOa 



CaO 



MgO 



K2O 



NagO 



P2O5 

 CO2 



Water lost below 100° C. (HgO-) 

 above 100° C. (H2O + 



A. 



B. 



Per Cent. 



Per Cent 



43-62 



47-61 



16-75 



17-47 



1-83 



3-44 



9-36 



8-78 



0-22 



0-13 



0-42 



0-87 



10-23 



7-04 



7-96 



2-49 



1-60 



1-60 



4-73 



4-49 



Trace 



1-10 



0-28 



0-28 



1-72 



0-84 



0-84 



3-71 



99-56 



99-85 



Specific gravity . . . . . . . . 2-83 2-68 



The first analysis shows a close approximation to augite-camptonite ; the rock 

 appears under the microscope as even-grained and holocrystalline. It is largely 

 composed of feldspar laths ; much granular augite, frequently with a violet tinge, 

 and distinctly pleochroic ; decomposed olivine, with selvage and seams of limonite ; 

 and a good deal of magnetite. Occasional apatite needles are present. These 

 phenocrysts are of labradorite, with an occasional augite, which does not exhibit 

 idiomorphic outline. There are also numerous cavities in the rock filled with 

 secondary carbonates. 



The second analysis is from a rock of more acid character, and is typical of 

 the basic porphyrites. The groundmass is composed of feldspar laths, granular 

 violet augite, much fine magnetite, and needles of ilmenite and apatite. The augite 

 is largely decomposed, with formation of chlorite and ilmenite. A good few pheno- 

 crysts of andesine occur (fig. 14, iv). 



The dolerite dykes were only observed penetrating the lower basic series at Mus- 

 grave Peninsula, but no doubt they occur in other parts of the area. The rocks 

 are generally coarse-grained, with conspicuous phenocrysts of augite, olivine, and 

 sometimes feldspar. Under the microscope the augite phenocrysts are usually 

 slightly pleochroic, the olivines altered to serpentine and limonite. The feldspar 

 is labradorite. The groundmass is usually fairly coarse, and is composed of 

 twinned laths of labradorite and grains of titaniferous augite and magnetite. 

 Comb-like forms of ilmenite (titaniferous magnetite) occur frequently in some 

 specimens. 



