32 
most contacts of porphyry and greenstone are particularly well hidden 
in this way. 
The Keewatin rocks, which underlie the larger part of the district, 
consist principally of rhyolites, basalts, and fragmental rocks. Cutting 
these Keewatin rocks are dykes or sills of gabbro, 1 whose courses in a 
general way correspond with the east-northeast strike of the volcanic 
rocks. 
The syenite porphyry, in the form of a stock-like intrusion, about 
1 • 5 miles from north to south, and 1 mile from east to west, cuts all the 
above-mentioned rocks. Numerous dykes of the porphyry cut the Keewatin 
rocks in the neighbourhood of the main mass, but rarely attain a width 
of more than a few feet. Such dykes appear to be particularly abundant 
on the northwest and south borders of the intrusion. Going northwest 
one passes within a distance of half a mile or more from massive syenite 
porphyry over areas that consist of 50 per cent or more porphyry in the 
form of dykes in the Keewatin, to places where only a few dykes occur, 
and finally to continuous Keewatin. The syenite porphyry, in turn, 
is cut by at least one large dyke of the so-called Later Gabbro of the region, 
which in this case approaches a true diabase. According to Cooke 2 all 
these intrusive rocks are pre-Huronian, that is, they do not cut the Cobalt 
conglomerate, which is well developed a few miles to the south, and they 
do cut the Timiskaming series. The only evidence obtained by the author 
is that the syenite porphyry is post “Older Gabbro” and pre “Later 
Gabbro.” 
DESCRIPTION OF SYENITE PORPHYRY 
Within the main mass the rock varies from acidic, light-coloured 
material to basic, dark green to black varieties, and from an extremely 
fine, dense mass to a very coarse-grained rock with brown feldspar pheno- 
crysts, many of which attain a length of several inches. The acidic vari- 
eties are highly feldspathic and as a rule contain free quartz; the basic 
hold abundant ferromagnesian constituents and quartz is scarce or lacking. 
The various phases almost invariably exhibit intrusive contacts with 
one another. Gradational changes, such as would result from differentia- 
tion after intrusion, are conspicuously lacking. The earliest phases are 
dark green, fine-grained to porphyritic, and, in some cases, of trachytic 
texture. The ferromagnesian minerals are present in amounts ranging 
from about 25 per cent to 60 per cent. The successively younger phases 
are lighter and lighter in colour, the ferromagnesian constituents decreasing 
in amount and feldspar becoming more abundant. The final phases are 
fine-grained dyke rocks composed almost altogether of feldspar (albite) 
and quartz, and they are accompanied by stringers and irregular areas of 
blue or green amphibole. 
The most basic phase is a fine-grained, very dark green rock. In the 
hand specimen, on freshly broken surfaces, it appears to consist largely of 
ferromagnesian constituents. On weathered surfaces tiny white grains of 
feldspar, about one millimetre in diameter, are visible, and the rock assumes 
a lighter colour. The ferromagnesian elements form somewhat more than 
50 per cent of the rock and are principally biotite and a dark green or black 
pyroxene. 
1 Cooke, H. C.: Op. cit., p. 79. "Older Gabbro." 
•Op. cit. 
