7-t Tlie American Geologist. Aui,'ust, i89i 
erii soils is a merely superficial phenomenon, being most strongly marked 
at the surface, and s:radiially elianging to yellow at a moderate depth. 
This fact, If fully established, will strongly corroborate the view here 
proposed, that the solar heat is the principal cause of the dehydration 
of the ferric oxide. 
Nearly all soils originate directly or indirectly, in the decay of the 
silicate minerals of the crystalline rocks, in which the iron is very laraely 
in the ferrous state. And it is well known that the meteoric waters 
percolating through the rocks not only introduce the carbon dioxide, 
which is the chief agent iu the kaolinization of the anhydrous silicates, 
but also the free oxygen required for the peroxidation of the iron. The 
sedentary soils of the South show very plainly also that the second pro- 
cess does not keep pace with the first ; for, while the superficial soil 
exhibits the brilliant colors of the ferric oxides, in the lower portion, 
which shades off insensibly into the underlying rocks, grayish and bluish 
tints prevail, indicating that the iron is still chiefly in the ferrous state. 
Hence the normal vertical order of colors in sedentary detritus seems to 
be as follows, beginning at the base : 1, Bluish, grayish and neutral tints, 
due to ferrous oxide ; 2, the yellow and brown tints of the ferric hyd- 
rates : and, 3, in warm countries, the red resulting from the dehydration 
of the ferric hvdrates. 
The foregoing embodies the chief points of ni}- previous paper, 
which was based very largelj' upon personal observations made 
some twenty j'ears ago while I was engaged in mining operations 
in North Carolina and Virginia, and enjoyed unusually* favorable 
opportunities for observing fresh and normal sections of the sed- 
entary soil. Later, also, I traveled extensivel}' in these and other 
southern states ; in the West Indies, from Cuba to Trinidad and 
the northern coast of South America ; and in Europe, from Sicilj' 
to Scandinavia — giving particular attention everywhere to the 
colors of the soils. That paper was published in the hope that it 
would call out the observations and views of other geologists ; but 
it appears to have borne no fruit of that kind until the appearance, 
a little more than a year ago, of the comprehensive and valuable 
monograph b}- I. C. Russell on the " subaerial decay of rocks and 
origin of the red color of certain formations,"* to which the 
author has appended an important bibliographic list. In the first 
part of this essay relating to the subaerial decay of rocks, 
Attention is directed to the widespread decay of the surface rocks 
throughout the Appalachian region, south of the southern limit of the 
glaciated area of northeastern America. It is shown, also, that rock- 
decay is far more advanced in the southern than in the central and 
northern Atlantic states, and increases gradually southward. This vari- 
»Bulletin No. 53 of* the United States Geological Survey, Washing- 
ton, 1889. 
