20 
IOWA ACADEMY OP SCIENCE 
zone and the precipitation of calcium carbonate is rarely opera- 
tive. This seems positive from the presence of limestone peb- 
bles, in the drift wherever the matrix reacts to acid. (7) The 
deposition of lime carbonate in an old leached zone by waters 
which have percolated down from an overlying calcareous for- 
mation is also not usually evident in clean exposures. Theo- 
retically the conditions for precipitation in such cases are ex- 
ceptional. (8) The phenomenon of leaching is one of the prac- 
tical and legitimate criteria for the age of drift sheets or for 
differentiating drift-sheets whose general conditions are com- 
parable to those of the east half of Iowa. Like all other cri- 
teria it 'must be used guardedly. Careful records of all the 
facts should be made. 
In addition to the foregoing discussion, the writer points 
out the need of discriminating in the use of such terms as oxi- 
dation, leaching, decomposition, staining, and to avoid as much 
as possible such general terms as weathering, alteration, etc., 
without specific definition. 
Julius Kosenwald Hall, 
University oe Chicago. 
