38 
tively small amount. In one case, a large, fibrous plate of blue amphibole 
has cut directly across an area of brown biotite, and the perfect cleavage 
lines of the latter can still be seen cutting across the cleavage of the amphi- 
bole. It is extremely difficult to avoid the conclusion in these cases that 
soda has been concentrated in the final stages of intrusion and has found 
expression in the development of albite and blue amphibole. 
In this particular rock there is also good evidence of the replacement 
of the original potassium-feldspar phenocrysts by albite. Some quartz 
has evidently been introduced at about the same times as the soda-feldspar. 
Titanite and apatite, the latter in many cases very coarse, are common 
accessories in the rock. 
TRACHYTIC TYPE 
The trachytic type of syenite porphyry is quite abundant. Pheno- 
crysts of orthoclase and microperthite, up to three-fourths of an inch in 
length, form about 50 per cent of this rock; they are in parallel orientation. 
Between the grains of feldspar lie large grains of aegirine-augite, showing 
twinning, and numerous large plates of brown biotite. Apatite and titanite 
are abundant in coarse grains, and magnetite is more sparingly present. 
A small amount of albite is present, mostly as rims around the larger 
grains of potassium-feldspar. The aegirine-augite is in many instances 
altered to the blue-green amphibole, with a small extinction angle. 
BASIC TYPE 
The most mafic variety of rock encountered consists of about 50 
per cent ferromagnesian constituents. Pyroxene, in idiomorphic grains 
up to one millimetre in length, predominates. This colourless to light 
green mineral has an extinction angle of 54 degrees and its index is approxi- 
mately 1 • 70, thus being confirmed as augite. Apatite occurs as large crystals. 
Biotite, containing fine needles that are presumably rutile and numerous 
pleochroic haloes probably due to zircon inclusions, is common. A rela- 
tively small amount of finely crystalline groundmass consists largely of 
albite, with some orthoclase and very little quartz. 
In this rock the augite, apatite, and biotite have been corroded and 
replaced by the groundmass. This phenomenon is apparently quite char- 
acteristic of minerals which have crystallized in alkaline magmas. 1 
CONTACT EFFECTS 
Some interesting features occur where the more basic varieties are cut 
by small dykes of the more acid type. In numerous places dark coloured 
varieties of the syenite porphyry, more or less trachytic in texture, are cut 
by brown, coarsely crystalline stringers of the more acidic types. One 
thin section, across such a contact, shows that the stringer consists of large 
phenocrysts of feldspar, principally microperthite, many of which have 
rims and hold irregular areas of clear albite. Aegirine-augite is very spar- 
ingly evident, as also is the blue amphibole described above. Titanite and 
magnetite or ilmenite are present in small amount. 
' Adams, F. D., and Barlow, A. E.: Geology of Ha 1 i burton and Bancroft Areas”: Geol. Surv., Canada, Mem. 8, 
p. 232 (1910). 
