1 70 The American Geologist. March, i89o 
tiire drainage system, a well developed erosion topography 
and a change of color from blue at the base to yellow in the 
upper portion. The exposures in southern Iowa usually show 
in addition a decrease in lime and an increase in rotted bowl- 
ders toward the surface with the almost invariable development 
of what is called the ferretto* and the occasional presence of 
a forest bed or series of water laid deposits between the drift 
and the loess. Of these phenomena the absence of lime and 
presence of ferretto are most widespread and most easily recog- 
nized. The drift is very largely made up of mechanically 
prepared material. The finer parts consist of broken and 
finely ground rock. Inasmuch as the glaciers passed over 
vast areas of limestone, a fresh drift normally carries large 
quantities of crushed limestone. This when touched by any 
of the common acids has the property of effervescing. When 
limestone is exposed to weathering agencies the soluble ma- 
terial is carried of? and that which remains is unacted on by 
ordinary acids. When a fresh drift containing small bits of 
limestone is exposed for a long time to weathering agencies, 
the same process takes place. After a time all the soluble 
])art of the limestone particles is carried away and the drift 
shows no reaction to the acid. It is thus possible normally 
to distinguish between an old, long exposed drift and a fresh 
one and in southern Iowa it has been found that before the 
loess was laid down the Kansan drift was so long exposed 
to the agencies of solution that there is no reaction to the 
acid at its upper surface and only a feel)le reaction to depths 
of five to nine feet below. On the other hand the younger 
drift sheets with the rarest exceptions show an elTervescence 
up to the very grass roots. 
Practicall}' all rocks carry a greater or less percentage of 
iron. The amount, while small, is usually the determining 
factor in the matter of color. As commonly found in the 
rocks iron exists in four forms: the carbonate (FeC03=Fe 
48.27 per cent which effects various shades of blue; limonite 
(FeO 3H=0=Fe 59.89 per cent) and the various earth\- 
(Khers which vary in shade from yellow to brown; hematite 
which (Fs03=Fe 70. per cent) in the pulverized form is red, 
and magnetite (Fe 3O =Fe 72.5 per cent) which is black. 
*Proc. Iowa Acad. Sci., vol. V, p. 90. 
