Spencer.] Oi4: [March 18, 



known to be removed everywhere to a depth of 100 feet, and in its eastern 

 portion to more than 360 feet, and still nearer Lake Ontario to a measured 

 depth of more than 300 feet below its waters. 



III. Topography and Hydrography of Lakes Superior, Michigan, 



Huron and St. Clair. 



As the origin of all our great lakes is so closely related, it will not be out 

 of place to describe briefly some of the features of the upper lakes that ap- 

 pear most striking. In the present paper it is only intended to call atten- 

 tion to some of the existing physical features of these great basins of water 

 that appear to show a relation which existed in a more or less common 

 origin of all our lakes. Though I have frequently visited many localities 

 on these lakes, for this portion of the present paper I am particularly in- 

 debted to General Comstock, Superintendent of the U. S. Lake Survey, 

 who kindly furnished me with the lake charts. 



Lake Superior. This lake may be described as a large basin with a 

 level or gently undulating bottom and steep margins. The mean depth 

 may be placed at 800 feet below its present surface. Very few soundings 

 exceed 900 leet. Of these, one near the centre of the basin is 954 feet, and 

 another, not far from Duluth, is 1036 feet — the maximum depth of the 

 lake, as shown by the hydrography. 



The depth of the lake at three or four miles from the shore is generally 

 as great as in its centre ; in fact, it is often deeper near the shore on its 

 north-western side. However, about the Apostle islands, between the Pic- 

 tured rocks and St. Mary's river, and in some of the bays, the waters are 

 shallower than in the open lake, with their floors more or less gradually 

 sloping as they recede from the land. As is w^ell known, the lake is gener- 

 ally surrounded by crystalline or metamorphic rocks, which rise from sev- 

 eral hundred to even twelve or thirteen hundred feet above its surface. In 

 short, the near shore hydrography simply shows that the present sub- 

 merged margins of the lake are composed of the bases of the same rugged 

 hills that surround it above the water. The margin of this mountain- 

 bound basin forms a strong contrast with its floor, which, at most, is only 

 a slightly undulating plane, extending for nearly its whole length and 

 often for little less than its breadth, excepting in its south-eastern portion 

 and some other places referred to above. In fact, the lake bottom is quite 

 as level as most extensive planes which are now subjected to sub-aerial 

 action. 



That this great plane is not covered with any great depth of drift-deposit 

 (excepting locally) appears evident on examining the character of the 

 bottom of the lake in the soundings just off" Keweenaw point, and those to 

 the northward. In the various localities hard and rocky bottoms are alike 

 found in both places, at the same depths, so frequently that they cannot 

 be regarded as only rocky eminences protruding through the silt covered 

 bottom which is generally observed. 



The general direction of the deepest channel, for more than 300 miles 

 along the north-western margin of the lake, appears to point to a river 



