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1898-99. | THE GEOLOGICAL HISTORY OF LAKE SUPERIOR. 47 
formerly drained these lakes or connected them with one another. We 
are indebted mainly to Professor J. W. Spencer for the discovery and 
location of these connecting channels. One of them, starting in Lake 
Michigan, runs eastward across the State of the same name, through the 
basin of Lake Huron to the northern extremity of the Indian Peninsula, 
thence down the bottom of Georgian Bay and across the neck of land 
to Lake Ontario. Another of these buried channels runs northward 
from Lake Erie to the western extremity of Lake Ontario. The basins 
of all the lower lakes are excavated in the softer portions of the almost 
horizontal palzozoic strata, so that their shapes and directions conform 
to some extent with the geological structure or arrangement of the strata. 
Lake Superior lies in a hollow almost surrounded by the primitive 
crystalline rocks. At one time it was probably filled with newer strata, 
which have been mostly removed by aqueous and glacial denudation ; 
but small areas of those rocks still remain. 
Lake Superior lies in the region of the general watershed or summit 
level of the continent east of the Rocky Mountains, although it is only 
600 feet above the sea. From near its shores the water flows west to 
the Winnipeg basin, north to Hudson’s Bay, and south to the Mississippi, 
while its own discharge is eastward to the St. Lawrence. 
The drainage system, or catchment basin of Lake Superior proper, 
is consequently small, but it has a sort of extension or appendix in the 
basin of Lake Nipigon, which is really the uppermost of the Great 
Lakes of the St. Lawrence. This body of water measures eighty miles 
from north to south by fifty miles from east to west, with extensive 
bays on all sides. Its area is about half that of Lake Ontario, or 
between 3,000 and 4,000 square miles. Thirteen rivers and many 
brooks flow into it from various directions. One of its bays is only 
twenty-four miles from the nearest point of Lake Superior, and its 
surface has an elevation of 244 feet above the latter. The Nipigon 
river, which is the upward continuation of the St. Lawrence, is a clear- 
water stream and altogether the largest one flowing into Lake Superior. 
The larger number of the rivers and brooks falling into the great lake 
are darkly colored. Some of them look almost black as they enter its 
limpid water, the contrast being very striking. But the vegetable 
matter, to which the colour is due, soon becomes oxidized and disappears. 
There is a general absence of mud and also of dissolved mineral matter 
in the tributaries of Lake Superior, and hence its waters are not only 
singularly transparent, but also nearly chemically pure. The sounding 
lead has shown that the bottom of the Lake in nearly all parts consists 
of clay. 
