On the Contrast in Color. — Crosby. 81 
yellow ores of iron are found, as a rule, onh' in the later rocks ; 
while the red ores are generally restricted to the earlier rocks. 
This genetic relation of the yellow and red ores is one of the most 
familiar and generally accepted facts in geology. However recent 
the origin of the red ore (turgite or hematite) may appear to be in 
any case, we naturally infer that it was first 3'ellow, and that it 
has passed slowly or rapidl}*, as the case may be, but gradually, 
through the series of yellow hydrates. 
The frequent absence of any apparent cause for this change 
leads us to assume that it is essential!}- spontaneous, in the sense 
that, although often hastened by heat or other extraneous agency, 
it would take place eventually without such aid. This view is 
strengthened by the analogous series of changes exhibited b}' 
silica. The gelatinous silicic h3'drate obtained in the labratory or 
seen in the waters of thermal springs, loses water and hardens 
spontaneoush', and eventuall}- reaches the comparativeh' stable 
condition of opal, which is comparable with limonite ; and since 
opal, like limonite, is alwa^'S of recent origin, we know that it 
must change more slowly into anhydrous silica or quartz, as 
limonite changes to hematite. The aluminum and other hydrates 
manifest a similar tendency. As the dehydration continues, there 
is a concomitant change from the amorphous to the crystalline 
state, analogous to that observed in the devitrification of glass 
and obsidian ; and the dehydration is probably as spontaneous 
as the ciystallization. 
If it be conceded that the dehydration is virtually, if not abso- 
luteh', spontaneous, and there is no apparent alternative, it fol- 
lows that the color of a deposit, so far as it is due to ferric oxide, 
is, other things being equal, a function of its geological age. In 
other words, the color natui-ally tends with the lapse of time to 
change from 3-ellow to red ; and, although this tendenc}* exists in- 
dependently of the temperature, it is undoubtedly greatlj^ favored 
b}' a warm climate. Applying this principle to the sedentary soil 
of the South, we find that the superficial portion is red, not alone 
because it is exposed to a higher temperature than the subjacent 
yellow clay, but also because it is the oldest part. On the other 
hand, the limited occurrences of postglacial sedentar}- detritus in 
the North are, in the absence of the favoring climatic influence, 
still too young to exhibit the change of color even superficially-. 
It is generally conceded that the glaciated area was, in pregla- 
cial times, covered with a continuous sheet of sedentary detritus 
