80 The American Geologist. August, isoi 
On passing from the Piedmont district of North Carolina to tlie 
Blue Ridge and the t:ii)lehuid he3ond, the red clay, through the 
accelerated erosion, rapidly disappears, except in the most pro- 
tected places. The outcropping ledges and crags are bordered by 
successive and perfectly blended zones of disintegrating rock, and 
gray, gra3'ish yellow, and yellow clays ; and onh', as a rule, on 
the gentlest declivities are orange and red tints observed. The 
same conditions were observed in the Appalachian valley, between 
the Pennsylvania line and the Natural Bridge, yellow and orange 
being the prevailing colors, and red rarely occurring in consider- 
able patches. Although the comparative absence of the red cla^' 
in these more elevated and mountainous areas might be attributed 
to the colder climate, the correlation with the accelerated erosion 
appears on the whole more simple and direct ; and the latter must 
certainly be regarded as at least the chief cause. 
Although it appears unnecessar}- to materially modif}' my pre- 
vious statements concerning the distribution of the colors in sed- 
entary detritus, I am, as already stated, less disposed than formerl}' 
to insist upon the entire adequacy of the warmer climate of the 
South as an explanation of the red surface soil of that section. 
The dehydration of the ferric oxide is not wholly dependent 
upon heat or pressure or any obvious extraneous agency, but it is 
in a large degree, apparently, a spontaneous process. . Of this we 
have abundant evidence in nature and in the laboratory. When 
the iron, which exists in the various silicate minerals chiefly in the 
ferrous state, is liberated and peroxldized during the decay of these 
species, it combines naturally with a very large and indefinite pro- 
portion of water, forming the yellow hydrate which is seen as a 
flocculent or a gelatinous colloid in the waters of springs, bogs, 
and marshes, and when the hydrate is obtained as a precipitate in 
the laboratory. But this colloid mass, even if immersed in water 
and entirely undisturbed, gradually and spontaneously gives otf a 
lar"-e part of the water which the ferric oxide has so greedily 
absorbed when in the nascent state ; and it appears thus, as it 
slowly solidifies and hardens, to pass in succession through the 
forms of the various native yellow hydrates — limnite, xantkosi- 
derite and limonite, to gijthite. That this progressive change 
continues is evident from the fact that these yellow hydrates are 
o-radually replaced in the older formations by the red hydrate 
(turgite) and by ferric anhydride (hematite). When occurring as 
orio-inal or contemporaneous, and not as secondary, deposits, the 
