55 



On the Physical Geography, Geology, and Commercial Re- 

 sources of Lake Superior* By J. J. BlGSBY, M.D., late 

 British Secretary to the Canadian Boundary Commission, 

 &c. Communicated by the Author. \ 



I. Physical Geography . 



Lake Superior is included between W. longitude 84° 18' and 92° 

 19' — and N. latitude 46° 29' — 49° Y. It is to the east of, and 

 near to, the swell of high land which, stretching from the Rocky 

 Mountains to Lake Superior, in wide undulating plains, divides the 

 waters of the Mexican Gulf from those of Hudson's Bay ; — and 

 then, bifurcating, one fork proceeds on the north side of Lake Superior 

 eastward towards Labrador, in groups of broken hills, while the 

 other fork passes south-east as a rough and high country into the 

 lowlands of the United States. It therefore occupies an oblong 

 crescent-shaped hollow, with a general direction rather to the north 

 of east. It has literally thousands of lakes on its north, and hundreds 

 on its immediate south. It is 1750 miles round, 420 miles long, 

 and 163 in extreme breadth. It is 597 feet above the Atlantic. 

 Its greatest known depth is 792 feet. Soundings of 300, 400, 600 

 feet are common ; but extensive shallows and flats prevail in parts. 



The hydrographic basin of Lake Superior is singularly small, 

 particularly on the south shore, where the tributaries of the River 

 Mississippi and Lake Michigan often approach within 5 and 10 miles 

 of the lake. It seems to be its own fountainhead. 



The water is clear, greenish, extremely pure, pleasant to the 

 taste, and soft from the nearly total absence of limestone from these 

 regions. An imperial pint only contains smooth P ar *t or a grain of 

 mineral matters — carbonates of lime and magnesia, sulphate of lime, 

 peroxide of iron, and the oxide of manganese. 



The average annual temperature of the water is 40° F. ; being 

 about the same as that of the ocean at certain great depths. In 

 June, the lake is often covered with ice ; and in the middle of July, 

 the surface-water freezes in the morning — with patches of snow in 

 the clefts of the rocks. At this period of the year, or a few days 

 later, the smaller lakes on the north are steadily at 72° and 74° F. 



Lake Superior is not undergoing secular drainage. It is lowest 

 in April, and highest by a few feet, in September. The great annual 

 variations of rain of these countries produce corresponding changes 

 of level. There are no tides, and no cycle of years for lake-levels. 



Barometric changes produce curious local oscillations of level. 



* The statements in this communication are partly derived from the able 

 reports and charts of Messrs Bayfield, Logan, Foster, Owen, and others in the 

 service of the Governments of Great Britain and the United States, Dr Bigsby's 

 own researches on the northern shores of the Lake, for 440 miles, having sup- 

 plied the remainder. 



T Read in the Royal Institution. 



