a i 
Geology of Lake Superior. 5 
will be best seen by reference to a map, and will explain the 
reason fur measuring its length in the manner which we have 
just mentioned. 
3. The variation of the magnetic needle varies from 
2° 42’ to 12” & east. The dip, as ascertained by Captain 
Franklin, at Fort William is 77° 58’; the variation near 
that place is 9° 5’ east, ‘The needle is very much disturb- 
edin many places by the magnetic nature of the oxides of 
iron which euter into the composition of many of the rocks. 
The variation eucreases gradually from the eastern extre- 
mity of the Lake westward, butis greatest near the Grand 
Portage and Iie Royale. 
4. The water of Lake Superior isas pure and clear as 
any io the world, and is always extremely cold, from its 
great depth, which varies from SO to 150 fathoms, but as 
we could sound insuch depths only in very calm weather, 
not being furnished with the proper sounding machines, and 
did not sound in the centre, we have reason to think that its 
greatest depth will not be overrated at 200 fathoms. 
The wature of the bottom of this lake was frequently as- 
certaiued by us, as portions of it were always found adhering 
to our anchors and sounding icads. It consisted, for the 
most part, of a very adhesive clay, containing small shells, 
of the species at present existing in the lake, When ex- 
posed to the air, this clay immediately became indurated in 
so great a degree as to require a smart blow to break it.—It 
effervesced slightly with diluted witrie acid, but whether 
from a mixture of carbonate of lime, or from the presence of 
finely comminuted shells we could not determine. This clay 
is of different colours in different parts of the lake. Be- 
tween Lle Royale and the north shore it is blue in the vicini- 
ty of the south coast red, from the red oxide of iron in the 
sand stone of that coast: whilst near Michipicoten it is white 
and hardens into a substance resembling pipe clay, 
We have mentioned these circumotances Lecause they seem 
to 
