Boult. — Occurrence of Gold at Harbour Cone. 441 



In this mass of augite and plagioclase are large plienocrysts of 

 augite, often quite idiomorphie, and showing the usual cleavage 

 and characters. Olivine is also present throughout the rock 

 in rounded grains and large rounded plates, but is not remark- 

 ably abundant. It is quite undecomposed, and is distinguishable 

 from the augite with difficulty with ordinary polarised light, 

 but under crossed nicols it shows a much higher birefringence. 



A residuum of glass is present interstitial ly with the plagio- 

 clase laths. It is not present in great quantity, and is filled 

 with small colourless beads. In places these unite to form 

 colourless needles (margirites), and elsewhere to form dark- 

 coloured rods which appear as small dark needles piercing the 

 glass under the low-power objective. 



The coarse crystallization and almost entirely noncrystal- 

 line characters of this rock ally it closely to the dolerites. The 

 feldspar laths are much larger than are usually found in basalts 

 and more typical of dolerites. The residiuum of glass and flow 

 structure, however, class it with the basalts. The coarse struc- 

 ture is doubtless due to the fact that it has solidified in a pipe, 

 the exposed portion now, perhaps, being originally many hundred 

 feet deep. The flows from this pipe were no doubt much finer 

 in their crystallization. 



Under " General Geology " reference was made to a boulder 

 found in the mine valley with peculiar petrographical characters 

 (Plate XI, fig. 5). Its mineral constituents find no parallel 

 among any rocks that are described in any of the standard 

 works. In hand-specimens the rock is fresh and very dark, 

 showing plienocrysts of augite in places, and small shining 

 facets of a dark mineral under a magnifying-glass. Under the 

 microscope it is seen to be holocrystalline and prophyritic. The 

 phenocrysts consist of long polysynthetically twinned plagio- 

 clase with a maximum extinction on the albite plane of 19°, 

 thus being andesine. Bounded plates of augite are scattered 

 through the mass, and in places these are quite idiomorphie. 

 and show unbroken crystalline outline. They are very pale 

 green in colour, and non-pleochroic, with high birefringence. 



Enclosing these phenocrysts are a mass of irregular grains 

 of soda amphibole (probably hastingsite). They have a greenish- 

 brown colour, and are very pleochroic. These have needles and 

 small laths of andesine scattered throughout them, and form 

 two-thirds of the bulk of the rock. 



The order of crystallization has evidently been — (1) augite ; 

 (2) plagioclase phenocrysts ; (3) plagioclase in the ground-mass ; 

 (4) amphibole. This order of crystallization is a variation from 

 the normal, and somewhat suggests that of the diabases, though 

 in them the last mineral to crystallize is the augite, and horn- 



