Geology.'] 



SUBANTARCTIC ISLANDS OF NEW ZEALAND. 



691 



few zircons, usually small. Occasionally crystals of perofskite with a light-brown 

 translucence. Groundmass almost entirely feldspar microlites (anorthoclase ?). 

 There is some fine yellow dusty matter of secondary origin and some exceedingly 

 minute grains of magnetite. 



The rock occurs on the western shore of Garden Cove, at an elevation of about 

 200 ft. above sea-level. Numerous zircons have weathered out from its surface, 

 and can be found beneath the surface-covering of peat. 



Analysis. 





. 6512 





. 16-70 





400 





103 





1-25 





0-28 





0-80 





5-69 





3 24 





254 



SiOo 



Al.Og 



Fe^Og 



FeO ■ 



CaO 



MgO 



TiO., 



Na.,0 



K,0 



h;o 



10065 



Zirconia was not separated. 



This chemical composition is typically that of a trachyte. The structure of 

 the rock suggests that it forms a minor intrusion, but the geological relations are 

 completely obscured by the covering of peat. 



Trachyte. (Fig. 11.) 



Specimens of this type of lava were obtained from several localities. In ap- 

 pearance the rock varies but little. It is always dark green in colour, and shows 

 cleavage-surfaces of large crystals of feldspar. 



In section the feldspars are seen to be for the most part sanidine or anorthoclase, 

 both kinds of mineral being present generally in one slice. In some instances oligo- 

 clase occurs as well. Pyroxene is always present ; its green colour shows that it 

 is a kind of aegerine-augite, though the high extinction-angle shows that the aegerine 

 molecule is not contained in it to a large amount. In the groundmass the feldspar 

 crystals in many cases have an isometric habit, and the structure becomes ortho- 

 phyric. The pyroxene of the groundmass solidified after the feldspar ; it is in 

 irregular-shaped crystals (anhedrons) of a very pale green colour. There is a fair 

 amount of magnetite, and there are occasional pseudomorphs of serpentine after 

 olivine. 



In all the localities where this rock was found it was apparently the lowest of 

 the series of terraced lavas, and it may therefore be regarded as the earliest lava- 

 flow in this area. 



The rock was obtained from the following localities : Mount Honey, Mount 

 Eboule, Filhol Peaks, Mount Dumas, Mount Azimuth, and Mount Lyall. In every 

 case it was found at an altitude of about 1,000 ft. above sea-level. Some of the 

 dykes in Perseverance Harbour are of the same rock. 



