Bartrum.—Geology of Riverhead-Kaukapakapa District. 151 
The list given cannot be regarded as by any means exhaustive: many 
unobtrusive occurrences have doubtless been passed by unobserved. 
In reality the “ serpentines ” differ considerably in their true character 
one from another. The majority vary between wholly serpentinized dunite 
with subsidiary bastite, and rocks in which the bastite has increased so 
considerably in proportion to serpentine that the name “ harzburgite ” i 
towards dunite-serpentine. The dark rock is built of chondri of partially 
serpentinized olivine enwrapped poecilitically by a moderately refractive, 
colourless, altered mass, which is only faintly birefracting, and which is 
probably referable to saussurite, since there are occasional remnants of 
basic plagioclase associated with it. The troctolite is evidently a relatively 
acidic variation of the feldspathic peridotite, for the proportions of the 
saussurite and plagioclase to original olivine vary greatly in the sections 
examined. 
In dump-heaps of the same quarry there are frequent fairly coarse 
‘fragments of a white pyroxenite which consists in the main of two minerals ; 
the more important is a colourless monoclinic pyroxene, which from casual 
inspection seems to be diopside, and with it is a fairly large amount of 
allage. The full study of this and several others of the rocks has not 
yet been attempted. 
Quartz-porphyrite of Flat-top Hill. 
graphically the rock is an open-grained 
plexus of laths of plagioclase (andesine) which enwrap subordinate pale- 
porphyrite. As there is every reason to believe that the quartz is a 
pneumatolytic precipitate from the original magma, the rock has been 
classed as a quartz-porphyrite. 
