2 6o O. W. WILLCOX 



borne in mind that many of the innumerable sloughs of the younger 

 drift-sheets occupy the lowest points of depressions without outlet. 

 These depressions constitute isolated basins having an area varying 

 from two or three up to as many hundreds of acres in extent. The 

 floor of these basins consists of the finely pulverized rock-flour char- 

 acteristic of the ground moraine. Owing to its fineness, the material 

 of the till is in the most favorable condition to be attacked by the 

 agencies of weathering. The alkaline- earth carbonates are attacked 

 and rendered soluble by the carbon dioxide of the air; the sulphur 

 in the pyritiferous minerals of the till is oxidized and converted into 

 sulphuric acid, which ultimately appears in the form of its calcium 

 and magnesium salts. Doubtless a certain proportion of the mate- 

 rial dissolved out of the till enters the general underground circula- 

 tion, but a large part simply collects in the water which has found 

 its way to the lowest part of the basin and has there been concentrated 

 by evaporation. Since the final retreat of the ice, this process of 

 concentration has in many cases brought the percentage of dissolved 

 matter up to a figure quite beyond that which is usual for surface 

 water. This is especially true of small ponds which receive the 

 drainage of large areas; but even those located in small basins 

 frequently contain appreciable amounts of salts, as is apparent from 

 the white residue left on their floors when they dry up in summer. 

 Many of the lakelets of the drift are, on a small scale, embryo salt 

 lakes, in which magnesium sulphate instead of sodium chloride is 

 the principal dissolved substance. 



The earth in their vicinity is consequently saturated with water 

 carrying a notable amount of saline matter, chiefly soluble salts 

 of magnesium. When the farmer tiles out the basin, the excess 

 of water is drawn off through the drains. Much of the 

 dissolved magnesium is thus removed, but not all, for no drain 

 will at once remove all of the water; even the percolation 

 of the rain water of several successive seasons is frequently insuffi- 

 cient to wash out all of the deleterious salts. 1 Although these 



1 This will be understood by every student of chemistry who has had occasion to 

 wash a dense precipitate to free it from a soluble salt. The obstinate retention of 

 certain salts freely soluble in water by a finely divided solid is not always purely 

 mechanical. The phenomena of adsorption (adsorption is different from absorption) , 

 of which chemists have only recently begun to take account, come largely into play. 



