Bartrum.. — Rocks of Mount Gargill, Dunedin, 



175 



probable point of origin of basanitic pebbles found in gravels at Mount 

 Cronin. It is uncertain whether this supposed basanite is that recorded 

 by Professor Ulrich from the Mount Cargill area. 



A similar rock is found in the vallev of the North-east Vallev 

 Stream above where it strikes in a northerly direction away from its 

 previous course alongside the North-east Valley Road. 



After some difficulty, staining-tests made on these rocks showed a few 

 small crystals that may be nepheline. As, however, some undoubted 

 olivine had gelatinized and absorbed the stain, there is doubt as to this 

 identification. For this reason these rocks are only tentatively classed 

 as basanites. 



(a.) Mount Holmes Basalt. 



This is macroscopically an open-grained basaltic-looking greyish-black 

 rock, showing plentiful small crystals of olivine and augite, and weather- 

 ing out to a greyish-fawn colour. 



Microscopically it is holocrystalline, and of porphyritic, hypidiomorphic 

 structure. The chief phenocrysts are faint greenish-pink augite and olivine : 

 they are of large size. The olivine is very fresh and sharply edged. 

 Augite is frequently of a dirty-green colour ; it occasionally enwraps the 

 olivine. Tue augite also commonly includes magnetite, and in a few 

 instances a little feldspar. 



Feldspar and augite are both porphyritic. The feldspar typically is 

 much corroded, and has numerous inclusions of groundmass. Twinning 

 by the Carlsbad and albite laws is prevalent. The varieties varv from 

 andesine-labradorite to labradorite. 



The open fine-grained base which encloses the phenocrysts consists of 

 a plexus of well-shaped feldspar laths which enwrap plentiful microlitic 

 almost colourless augite granules, fairly abundant magnetite, a little 

 coarser olivine, and a little ilmenite. Crystallites of indistinct nature 

 occupy the finest interspaces between the augite granules. 



Chemical Characters. 



An analysis of this rock shows that it is a fairly typical basalt. The 

 percentage of ferrous iron is particularly high, and is probably due largely 

 to the greenish augite, as well as to the magnetite and ilmenite. 



An analysis of a basalt, quoted from Rosenbusch's " Elemente der 

 Gesteinslehre." is also appended. 



SiOo .. • 



A1 2 3 



Fe,0 3 



Feb 



MgO 



CaO 



K 2 



Na 2 



H 2 



Ti0 2 



99-63 100-46 



A. Basalt. Mount Ho'.mes, Mount Cargill, Dunedin. (Analysis. J. 

 Bartrum.) 



B. Basalt.* 



* Rosenbusch, " Elemente der Gesteinslehre," p. 323, No. 15, 1001 ed. 



