126 



water is shallow, at no place more thau 12 feet deep. The s?reater part 

 of its bed is muddy, and two-thirds of its outline is marshy. The ordi- 

 nary variation in the level of the lake during the year is less than two 

 feet. Such variation does not materially change the area of the lake or 

 appreciably modify the A^arious forms of life that inhabit its waters. 



Perhaps one-eighth of the outline of the lake is low ground, marshy, 

 swampy or boggy. But in general the muck or black mud is shallow, 

 seldom more than two or three feet in depth, and it rests on a bed of 

 hard sand or gravel. From the shore out to a depth of six or eight feet 

 the lake bed is of hard sand or gravel, even along the low ground. At the 

 mouths of the southeast and northeast inlets there are considerable areas 

 of shallow mud over the sand, and at the mouths of the lesser inlets 

 there is always a little soil. But for long distances along the steep banks 

 of clay or gravel there is no fine soil, just sand or gravel. On the north, 

 west and south this bed of sand and gravel supports an abundant growth 

 of Chara, which is generally of small size and thickly crusted with 

 calcic carbonate. This bed is also the home of immense numbers of 

 bivalve mollusks. The chara and shells of dead mollusks yield consid- 

 erable quantities of calcic carbonate. At first one would expect to find 

 this material making deposits over the bed of this shallow water. But 

 this calcic carbonate and other fine material is SAvept away and deposited 

 in deeper water, where it helps to form the extensive marl beds of the 

 lake. During the summer there are more winds from the east than from 

 any other quarter, but during the year there are more westerly winds, 

 and in general the westerly winds are stronger. There are also many 

 northerly winds and many southerly winds, so that during the year 

 there are numerous winds from each quarter. These winds pile up the 

 water along the shores toward which they blow. This causes more or 

 less of an undercurrent toward the deep water which caiTies with it 

 all the fine material of the shallower water. As the westerly winds 

 are more numerous and stronger these undercurrents ai"e stronger on the 

 east, carrying the fine material into deeper water, the marl beds com- 

 mencing in eight to ten feet of Avater instead of in six to eight feet of 

 water as on the other sides of the lake. The marl forms a rich soil which 

 shades off into darker material under deeper water. During the winter 

 ice forms to a thickness of from 15 to 25 inches. As the ice expands 

 it crushes against the banks with great force. Where the shores are low 

 the ice often pushes great quantities of sand and other materials up into 



