On the Contrast in Color. — Crosby. 75 
ation is thought to be due to climatic causes, combined with recent 
orographic movements which have accelerated denudation in the 
northern portion of the region under discussion. From the brief review 
of the geographical distribution of residual deposits in various regions, 
together with other considerations, the conclusion is drawn that rocks 
decay most rapidly in warm, humid climates. 
After describing the characteristics of the residual or sedentary 
clays of the South, including the color, Mr. Russell refers to my 
contribution as follows : 
The contrast in color between northern and southern landscapes in 
the Appalachian belt has recently been explained by W. O. Crosby . . . 
on the assumption that the observed difference of color has resulted 
directly from differences of temperature between the North and South. 
In the essay referred to it is urged that the higher temperature at the 
South is capable of dehydrating the ferric oxides impregnating the soils, 
thus changing their color from yellow to red ; and also that the red 
color of residual clays is a superficial phenomenon, confined to the 
immediate surface of the deposits. My own observations do not confirm 
these conclusions. At many localities in the Appalachian region south 
of Pennsylvania where residual deposits were observed, the character- 
istic red color was seen to extend far below the surface, and as a rule to 
reach the bottom of fresh exposures. In many localities the color of the 
residual clays at a depth of twenty or even fifty feet is similar, so far as 
the eye can distinguish, to the color in the same sections only a foot or 
two below the surface. The great depth to which the red color extends 
renders it evident that it cannot be directly dependent on solar heat. 
Again, over considerable areas in the South the surface clays are vari- 
ous shades of yellow, which would not be expected if the red color of 
adjacent fields is due to temperature. 
During the last six years I have gradualh' come to attach less 
weight than at first to the difference in temperature as a sole and 
sufficient cause of the difference in color between northern and 
southern soils ; but I still hold that it is an important factor in the 
entirely adequate explanation ; and so, apparenth', does Mr. 
Russell. In fact, he proves that the cause must be largelj' cli- 
matic b}' showing : ( 1 ) that the tj'pical residual claA^s of the South 
are usualh' red, and common to a great varietj' of geological for- 
mations ; and ( 2 ) that, as already quoted, they are conditioned in 
a very large degree b}' the combined action of heat and moisture, 
being but scantily developed in the arid regions of the South, and 
never with a red color under the humid climate of the North. 
Since the rocks from which the red claj'S are derived are almost 
never red, the redness is evidently incidental to the kaolinization, 
and its cause may therefore be looked for with much probability, 
at least, among the conditions favoring kaolinization, of which 
