1907-8.] THE GEOLOGY OF THE LAKE WENDIGOKAN REGION. 349 
of a porphyrite taken just east of the southwest corner of location HF 10, 
weathers brown on the surface to a depth of about half an inch. It is 
very compact and on the fresh surface is light grey in colour, and shows 
many crystals of pyrite. In thin section it shows a great deal of pyrite, 
some chlorite, very little hornblende, and large crystals of plagioclase 
badly decomposed. It has been suggested that the pyrite in these Kee- 
watin rocks is of secondary origin, having been derived from sulphates 
through reduction by carbonaceous matter. If there were life in the 
waters at that time, this is doubtless the best explanation for the presence 
of the pyrite in the sedimentary rocks, but it is much more probable that 
in these eruptive rocks the pyrite is pyrogenetic in origin. This pyrite 
is probably the source of the brown iron oxide which covers the surface 
of the rock. 
The amygdaloidal rocks which are most frequently found west of 
Tallon Lake are greatly weathered, light or dark green rocks, generally 
having their cavities filled with quartz or calcite. It is noticeable that 
calcite is very common among many of these older rocks, filling cracks 
and cavities, and its presence is readily detected by the action of acid on 
the rock. It might be that this calcite has been recrystallized from por- 
tions of limestone which at one time covered the greenstones and has since 
been weathered away. This was suggested by Professor Miller’s descrip- 
tion of the relation between the greenstone and limestone in Eastern 
Ontario. On the other hand it may be a much later deposit from percolat- 
ing waters. A thin section from the amygdaloid occurring a short distance 
from the southwest corner of location HF23 shows it to be a basic lava 
with many amygdules filled with chlorite resulting from the decomposition 
of minerals formerly contained in the cavities. Along the south side of 
HF-.6 a greatly weathered greenstone occurs which exhibits no character- 
istic features in the thin section. In location HF34 there was found a 
mixture of greenstone and green schist, some of the former in small patches 
showing the lava pillow structure. Near the southeast corner of HF 16 
there is a rock quite massive in appearance, weathering brown and con- 
taining a good deal of pyrite. In the thin section it exhibits a structure 
with small circular masses like that of the taconytes so frequently described 
and pictured in the Minnesota state reports. ‘Though it is greatly weath- 
ered the structure shows that it is a fragmental rock and of volcanic origin. 
It is a type of tuff and since it contains much iron carbonate it seems 
to be closely related to the carbonate schists described later in this article 
and these probably grade into one another. However, its massive struc- 
ture and much fresher appearance seem to separate it from the typical 
carbonate schists. 
