1907-8.] THE GEOLOGY OF THE LAKE WENDIGOKAN REGION. 345 
_ the eastern end of the district explored. It should be added that the 
figures given here for the different elevations were largely obtained by use 
of the aneroid barometer, and can, therefore, be only approximate because 
of the conditions under which the aneroid was used. 
A Historical GEOLOGY SKETCH. 
The earliest geological records found in this region are in the series 
cf extrusive volcanic rocks which correspond to the oldest known rock 
series of the continent. There was during the earliest period recorded by 
rocks, a great series of surface lava flows, as shown by the porphyries 
and by the amygdaloids found south of Wendigokan and again to the 
west on the shore of Lake Nipigon, where the pillow and brecciated struc- 
tures are also well developed in the greenstones. Along with the lava 
flows, there were probably deposits of ashes and other volcanic detritus 
which later became consolidated, forming taconytes, which may be seen 
along the south side of HF16. ‘These rocks are very common in some of 
the iron formations of Minnesota and are frequently described in the geo- 
logical reports of that state. While the surface materials were being 
extruded and possibly after they had ceased to be poured out, there were 
igneous. rocks cooling beneath the surface, where they remained until 
later exposed by erosion. ‘These rocks are now represented by the quartz 
diorites which are rather widely distributed in the region. 
After the great period of volcanic activity there appears to have been. 
a prolonged period with little evidence of vulcanism during which the 
rocks were subject to the action of air and water which decomposed 
them and carried the solutions leached out to the sea or inland waters. 
There is no direct evidence here that these waters were marine, though 
the pressure of limestone in some other places, which may of course be a 
chemical and not a marine organic deposit, is strongly in favour of this 
assumption, and the conditions which followed are simpler and more easily 
explained by considering that these deposits were made in the shallow 
arms of the sea or in partially or entirely enclosed bodies of sea water shut 
off by low barriers. The sea must be considered as covering the Wendi- 
gokan region before the next formation was deposited. 
On account of the presence of much carbon dioxide which is sup- 
posed to have been present in the air and waters at this time, the materials 
would be largely precipitated from solution as carbonates mixed with 
more or less mechanical sediments in the form of fine clastic material. 
It is quite probable that there was considerable precipitation while the 
volcanic, fragmental materials were being deposited, because of the 
