On the Contrast in Color. — Crosby. 79 
residual cla^s proves a general predominance of decay over denu- 
dation ; but even where the rate of deca}' is uniform, the ever 
varying conditions of erosion must give rise to ever}' phase be- 
tween the greatest observed depth of undisturbed residual clay 
with the full complement of colors — red, orange, 3'ellow, and gray 
— and the hard, bare ledges seen in crags and stream Ijeds ; and 
we may safel}^ predict that in passing from the one extreme of 
denudation to the other, these tints will, as a rule, appear in suc- 
cession at the surface. 
According to my observations, the surface of the stricth* seden- 
tary detritus of the Piedmont region is rareh* yellow, except 
where the conditions are obviously more favoraI:)le for rapid erosion 
than over adjacent red areas. Often in the same limited field it 
can be seen that the steeper slopes of the land, or, in general, 
those areas most exposed to the wash of the rains, are 3-ellow, 
while the more level or less exposed parts are red. The colors 
thus tend to distinguish the areas of slow and rapid erosion ; but 
it is intended, of course, to embrace in the latter, as in the former, 
only the general ablation of the surface, and not the gullies so 
characteristic of southern hillsides, which, when once started, 
quickly cut clean, vertical sections through the cla3's. Now the 
fact that the red clay washed b}- the rains from the steeper slopes 
must be spread, in large part, over the more level areas imme- 
diatel}^ adjoining, affords an obvious and simple explanation, not 
only of the constantl}' varying thickness of the red clay, but 
especially of the exceptionally great thicknesses sometimes 
observed. I have purposely- neglected to take account of these 
before, because the statement that the red clay is mainh' super- 
ficial was intended to apph* only to detritus that is still In sifu, or 
strictl}' sedentar}'. No argument is required to show that by rain- 
wash from surrounding slopes the i-ed clay might be accumulated 
upon a limited area to almost an}- depth, even fifty feet, as stated 
by Kussell. But it is an obvious mistake to compare such special 
accumulations of transported detritus, which are in general readily 
recognized by their situations and horizontal stratification, with 
clay which is still in situ. The same principle also explains the 
exposure of yellow clay over level areas ; for evidently when the 
red clay has been completely washed from the slopes, the yellow 
cla}' will experience a similar ablation ; and it is not ditlicult to 
see how the conditions would often be favorable to a commingling 
or inter-stratification of red and A^ellow cla3's. 
