Drift of NortJizvestern lozva. — Bain. 1 7 1 
Magnetite is rarely an important constituent of sedimentary 
though common in the igneous and metamorphic rocks. As 
will be seen from the chemical formulas the carbonate contains 
relatively the least oxygen, and of the three common forms 
the hematite contains the most, and limonite and the various 
earthy ochers occupy an intermediate position. If then a 
mass of rock whose color is determined by the iron content 
be subjected to oxidation the color will pass progressively 
from drab or blue through yellow and brown to red. The 
latter color is the indicating mark of a high stage of oxida- 
tion. Oxygen is one of the most active chemicals in the air 
and oxidation is one of the most widespread and prevalent 
processes to which rocks are subjected in weathering. In 
driftless areas, where the soil has been formed by the slow 
weathering away of the rocks red soils are common. In the 
driftless area of Iowa and adjacent states the hard blue lime- 
stone and dolomytes of the Palaeozoic have been leached and 
oxidized till a sticky red clay called geest, alone remains. In 
many of the southern states beyond the limits of glacial action 
red soils of this genesis are common. 
Where drift is exposed to atmospheric agencies the pro- 
cesses of weathering are closely akin to those which take 
]Dlace in the weathering of ordinary rocks. The chemical ac- 
tivities are relatively more intense as the material is already 
broken up and there is no need to wait for the slow processes 
of frost action to shatter the rock. The finely comminuted 
rock flour is at once attacked by chemical agents and decal- 
cification, oxidation and ferrugination at once set in. Old 
drifts accordingly soon become highly oxidized. The iron as- 
sumes a deep brown to red color. This color is less and less 
intense from the surface downward. The iron aggregates and 
to a certain extent sometimes cements the soil. This dark, 
iron-stained and highly oxidized band at the surface of the 
loess is what is called the ferretto. In southern Iowa it is 
widespread below the loess and at the surface of the drift and 
is interpreted as indicating a considerable period of weather- 
ing between the deposition of the two beds. 
The oxidation which produces the ferretto leads also to 
the breaking down of the bowlders at the old drift surface. 
It is a very usual thing to find the granites and other crvstal- 
