On the Contrast in Color. — Crosby. 73 
ern eyes, is the bright red color of the soil, and the general predominance 
of this color over the brownish and yellowish tints. This begins to be 
noticeable In the latitude of southern Pennsylvania, and becomes more 
and more marked as we cross Virginia Into the Carolinas ; while In the 
West Indies and South America the redness of the soil is even more 
intense and universal than in the southern states. So far as I have 
been able to learn by reading and inquiry, this difference in color be- 
tween the soils of high and low latitudes is more or less distinctly 
observable in all longitudes, and In the southern as well as in the 
northern hemisphere. 
The brown, yellow and buff colors, so characteristic of northern soils, 
are undoubtedly due chiefly to the yellow ferric hydrates, like gothlte, 
liraonlte, and xanthoslderite ; while the red color of southern soils, 
although commonly attributed to hematite, is probably in many, if not 
most cases due to the red ferric hydrate turglte. The main question, 
then. Why are northern soils yellow and southern soils red ? Is really 
equivalent to. Why is the ferric oxide in northern soils highly hydr 
ated ( gothlte, limonite, etc. ), while that in the southern soils is only 
slightly hydrated, (turgite), or anhydrous (hematite)? 
It is manifestly Impossible to answer this question by correlating the 
difference in color with a difference in tlie rocks of the two regions; for, 
while the red clays of the South are found on nearly all geological for- 
mations, they appear to have their best development on the primary or 
•crystalline rocks, and these are indistinguishable from the similar rocks 
of the North. But a satisfactory solution is, I think, found by correlat- 
ing the color-difference with the one physical feature upon which all the 
other contrasts between the North and South depend — the climate. In 
other words, the difference in color depends upon the difference in tem- 
perature. It Is well known to chemists that ferric hydrate, the coloring 
agent of northern soils, is dehydrated at the temperature of boiling 
water ; and It seems probable that a partial, if not complete dehydration 
may result at much lower temperatures, if unlimited or geologically long 
time is allowed. And, in this connection, it is Important to observe that 
the surface soils of the South attain, at times a high temperature, and 
that in both regions, but especially in the South, the detritus is, quite 
certainly, chiefly of preglacial origin. The detritus of the South, it is 
well known, is, except on the flood-plains of the streams, chiefly seden- 
tary, often retaining almost perfectly the structure lines of the rock 
from which it is derived ; while the debris covering the rocks in the 
North is almost wholly transported, consisting of the modified and 
unmodified glacial drift. Hence it is evident that the characteristic 
colors of the North and South are approximately coterminous with the 
sedentary detritus and drift. But it seems impossible to ascribe the 
color-difference to glaciatlon ; for wherever in the North we find seden- 
tary soils, either post or ante-glacial, as in the case of trap dikes which 
have been decomposed to a considerable depth below the surface of the 
Inclosing rocks, the colors are brown and yellew, never red. 
Although it seems not to have attracted general attention, my obser- 
vations show that frequently, if not always, the red color of the South- 
