Crosby.] 220 [Jan. 17, 



distinctly red. The only important exceptions are where the red 

 soil results from the disintegration of a red rock, or is itself geo- 

 logically old. Thus, the red color of the soil on the triassic 

 areas, and of the clays at Brandon, Vt., and Gay Head, does not 

 belong to the present or any recent period, but is due to the per- 

 oxidation of iron in triassic and tertiary times. On the other 

 hand, one of the most striking features of the scenery of the south- 

 ern states, especially for northern eyes, is the bright red color of 

 the soil and the general predominance of this color over the brown- 

 ish and yellowish tints. This begins to be noticeable in the lati- 

 tude 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 in- 

 tense and universal than in the southern states. So far as I have 

 been able to learn by reading and inquiry, this difference in color 

 between the soils of high and low latitudes is more or less dis- 

 tinctly observable in all longitudes and in the southern as well as 

 the northern hemispheres. 



The brown, yellow and buff colors, so characteristic of northern 

 soils, are undoubtedly due chiefly to the yellow ferric hydrates, like 

 gothite, limonite and xanthosiderite ; while the red color of south- 

 ern soils, although commonly attributed to hematite, is probably 

 in many, if not most, cases due to the red ferric hydrate, turgite. 

 I know of no way to determine whether or not any part of the 

 water which a red clay yields on analysis is combined with the fer- 

 ric oxide, but it seems necessary to suppose that a part of it is, 

 since the conditions are extremely unfavorable for the formation 

 or existence of ferric anhydride. The main question, then, Why 

 are northern soils yellow and southern soils red? is really equiva- 

 lent to, Why is the ferric oxide in northern soils highly hydrated 

 (gothite, limonite, etc.) while that in southern soils is only slightly 

 hydrated (turgite) or anhydrous (hematite) ? 



It is manifestly impossible to answer this question by correlat- 

 ing the difference in color with a difference in the rocks of the two 

 regions ; for, while the red clays of the south are found on nearly 

 all geological formations, they appear to have their best develop- 

 ment on the primary or crystalline rocks, and these are indistin- 

 guishable from the similar rocks of the north. But a satisfactory 

 solution is, I think, found by correlating the color-difference with 

 the one physical feature upon which all the other contrasts be- 



