212 TRANSACTIONS OF THE |aPR. 15, 
ters in length. They are not noticeably corroded. Strictly 
speaking, this rock is a camptonite, but its real affinities seem 
to be with the diorite-porphyrites. Spec. 608 also shows por- 
phyritic hornblendes, brown like those of the groundmass, 
along with the feldspar phenocrysts. 
Feldspar phenocrysts are present in all but one of the sec- 
tions. They are much corroded and kaolinized and surrounded 
by a rim of clear feldspar, probably secondary, at least later 
than the greater part of the groundmass. When not too much 
altered, they can be seen to be a very basic plagioclase with ex- 
tinctions up to at least 40°, indicating approximately bytownite. 
Twinning after the Carlsbad and Albite laws is observed, the 
latter polysynthetic. The crystals are strongly zonal towards 
the edges, the clear outer rim becoming rapidly more acid till it 
is an oligoclase with nearly straight extinction. The zonal out- 
side and decayed core are not always coterminous, the latter 
being often more or less zonal. There is no evidence that the 
clear outer rim formed later than the consolidation of the rock ; 
the decay of the feldspar within may well be due to its different 
composition. But it certainly formed after the greater part of 
the groundmass had crystallized out. 
The feldspar of the groundmass is composed in part of lath- 
shaped crystals, with the extinction angle of labradorite, zonal 
at the edges like the phenocrysts, but not corroded, though often 
decayed at the centre. The rest of the feldspar occurs as zonal 
rims to the labradorite rods and as irregular grains packed in 
among them. It is always fresh, and judging from its extinc- 
tion angles its composition varies from an acid labradorite to 
oligoclase or even albite. 
Quartz occurs as a phenocryst in spec. 336 and shows well de- 
fined crystal outlines, somewhat corroded, not mnch broken up. 
It occurs in the groundmass of all the dykes, though never in 
very large amount, and is then of the same age as the clear feld- 
spar forming the zonal rims and irregnlar grains, that is, the last 
constituent to crystalize. 
Magnetite and pyrite occur in small, well formed erystals. 
Apatite is rare. The hornblende of the concentrates appears to 
show transition to an ultramarine blue variety, probably a soda- 
hornblende. 
Flow structure is very well seen in some of these dykes, as 
shown in Pl. XIV., fig. 2. 
