1SS5.1 221 [Crosby. 



tween the north and south depend — the climate. In other words, 

 the difference in color depends upon a difference in temperature. 

 It is well known to chemists that ferric hydrate, the coloring 

 agent of northern soils, is dehydrated at the temperature of boil- 

 ing 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 espec- 

 ially 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 rivers, chiefly sedentary, often retaining al- 

 most perfectly the structure lines of the rock from which it is de- 

 rived ; 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 continuous with the sed- 

 entary detritus and drift. But it seems impossible to ascribe the 

 color- difference to glaciation ; for wherever in the north we find 

 sedentary soils, either post- or anteglacial, as in the case of trap 

 dykes which have been decomposed to a considerable depth below 

 the surface of the enclosing rocks, the colors are brown and yel- 

 low, never red. 



Although it seems not to have attracted general attention, 

 my observations show that frequently, if not always, the red color 

 of the southern soils is a merely superficial phenomenon, being 

 most strongly marked at the surface and gradually changing 

 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 largely 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 c*hief agent in 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 process does not keep pace with 

 the first ; for, while the superficial soil exhibits the brilliant colors 



