106 



ton station at the lower end of the middle section of the river 

 is fortunate in that it has enabled us to make collections in 

 water that is typical of the stream as a whole. No large trib- 

 utary is near enough to disturb the balance, and the greater 

 part of the water of the stream at the place of collection has 

 been moving in the channel for some time. There is, conse- 

 quently, abundant opportunity for a thorough mingling of the 

 waters derived from the various tributaries and for the breed- 

 ing of the plankton. The conditions are thus favorable for a 

 uniform distribution and development of the plankton in the 

 river at the place where our collections are made. 



THE RIVER BOTTOMS. 



As before stated, the present river channel, at least from 

 the bend near Hennepin to its mouth, lies in the bed of a stream 

 which formerly connected Lake Michigan with the Mississippi 

 River. The channel excavated by this ancient water-course is 

 far in excess of the demands of the present river, and it has 

 filled up by alluvial deposits, which constitute the bottom-lands 

 of to-day. This ancient channel varies in width from one and a 

 half to six miles, and borings made near Havana in the bottom- 

 lands of the present river, ten feet above low water, reveal a 

 deposit of alluvial soil twelve to eighteen feet in thickness 

 which lies above alternating beds of sand and fine clay. The 

 depth to which these beds extend is not known. Wells in the 

 bottom-lands are seldom deeper than fifty feet and usually do 

 not exceed twenty feet, water-bearing strata of sand being 

 found at varying levels between these limits. At Cedar Lake, 

 sixteen miles north of Havana, the pipe of a driven well was 

 checked at a depth of fifty-two feet, — forty feet below low water 

 and 389 feet above sea level, — apparently by rock, and the water 

 drawn from this depth is heavily charged with salts of iron, be- 

 ing similar to that from the shales of adjacent coal regions. 

 Alternating beds of sand, gravel, and clay similar to those found 

 beneath the alluvium in the bottom-lands were encountered in 

 a prospect-boring for coal at Mason City, twenty miles south- 



