708 WILLIAM C. ALDEN 
there was time for the reduction of the weathered drift to a residual 
state, such as seems to have been the case on the moderately mature 
erosion topography west of Rock River in southern Wisconsin, there 
is the usual condition of leaching of the clayey and sandy matrix 
to moderate depths and oxidation only to the degree of changing 
the original pinkish, bluish, or grayish color to a buff or brownish 
tint. 
It is thus evident that the apparent alteration of a single drift 
deposit by weathering and erosion may vary considerably in different 
and often closely adjacent parts of a single drainage basin in the same 
length of time, even where the differences in elevation are but slight 
and where the climate is the same. It is true that one cannot always 
see just what the particular relations were which resulted in the 
different degrees of alteration at two different exposures, and the 
observed differences are frequently not just what one would expect 
from reasoning along the line indicated. However, it is well to bear 
in mind the fact that such differences do occur where there appears 
to be no adequate ground for assigning the differently affected deposits 
to distinct stages of deposition. Caution must be used in applying 
the criteria and good and sufficient grounds for the differentiation 
should be observed. It is not necessary to enumerate these criteria 
in this paper. ‘They have been fully treated by competent authorities 
in other places. It seems to the writer particularly hazardous to 
assign a more recent age to what appears to be a thin and scattered 
deposit of fresh, unchanged, or little altered drift, exposed here and 
there, when the main deposit of the area in question is of similar litho- 
logic composition and has been considerably altered by weathering and 
erosion, unless a weathered zone, old soil, or vegetal deposit is observed 
clearly intercalated between the two drifts. It may not be easy to 
show that a weathered part has been removed by erosion at the 
particular places where the fresh drift is observed, or that the condi- 
tions for erosion, leaching, and weathering were such that no alteration 
of the upper part of the drift 7m satu was ever accomplished, but the 
burden of proof certainly lies with the interpretation which postulates 
distinct stages of glaciation. Differences in texture and conditions of 
deposition are particularly unsafe, since almost every conceivable 
variation of this kind may be found in different parts of what is 
