Leverett.] 
458 
[Jan. 1, 
depths, the variation depending largely upon drainage facilities. 
Where drainage is good and denudation moderate it is two to six 
feet, but beneath marshes there is little leaching below the black 
muck, such places having frequently marl beds at the surface of 
the till. In the level tract near Chicago (not marshy) leaching has 
in many places scarcely reached a foot in depth. Few analyses 
of the unleached tills of this region have been made from which the 
percentage of carbonates may be ascertained. Two analyses, by 
R. B. Riggs, 1 of a glacial and glacio-lacustrine till, near Milwaukee, 
Wisconsin, show about forty per cent, of carbonates. Three analyses 
of glacial sands from hillocks of angular gravel in Ohio, made by R. 
B. Smith, 2 show a wide difference in the amount of carbonates, one 
giving 70.364 per cent ; the second, 75.604 per cent, soluble in hydro- 
chloric acid, the greater part being in the form of carbonates of lime 
and magnesia ; the third giving 38.964 percent, of carbonates. My 
observations lead me to think that forty per cent, represents ap- 
proximately the amount of calcareous material in the till from 
which the buried soil is derived. There is, therefore, if we may 
reason from the few analyses made, and if we take no account of 
surface denudation which goes on more or less rapidly w T herever 
drainage is good, a probability that fully two-fifths of the upper 
five feet of the surface of the older drift sheet have been removed by 
leaching leaving the three feet of leached drift now associated with 
the buried soil. To accomplish this would certainly require the 
lapse of a long interval of time. 
The land surface south of the margin of the newer drift in Illi- 
nois, Indiana, and Ohio, is covered by two to five feet or more of 
silt, showing that it was extensively submerged after the soil had 
formed on the old drift surface. This silt is itself leached but has 
probably prevented the leaching from extending to greater depth 
beneath the old soil, it being very compact. 
The question may be raised whether the absence of carbonate of 
lime in the soil and subsoil and its presence in the underlying till 
of the older drift may not be due to original difference in structure. 
It is my own opinion based upon numerous comparisons of the soils 
with the till that the material forming the soil has the same origi- 
nal structure as the oxidized till, both being commingled drift con- 
1 Sixth Annual Report, U. S. Geological Survey, p. 250. 
2 Am. Journ. of Sci., May, 1884, pp. 383-384. 
