148 
IOWA ACADEMY OP SCIENCE 
to no groups smaller than those of the second order, although for conciseness 
subperiod is recognized. 
Based wholly upon provincial stratigraphy the next smaller group is the 
serial subdivision. Manifestly beyond the margins of a geologic province the 
geologic history must be perfectly independent. All attempts to make this sub- 
division a universal or even a continental time-group have thus far resulted in 
complete failure, and must necessarily continue to do so. 
The terranes are delimited lithologically, stratigraphically, or biotically; or 
in accordance with the composite determinations of all three criteria. In ter- 
ranal definition three other features are of great importance; these are geo- 
graphic extent, topographic expression, and economic content. Some of the 
Iowa terranes now recognized are no doubt susceptible of further subdivision. 
The formations here enumerated are those which must be, sooner or later, 
generally recognized in the state. Some of them are in Iowa very unimportant; 
but they find greater development in adjoining states. Many titles appear, 
indeed, to have been little used in Iowa. The names adopted have all passed 
the gauntlet of the most exacting synonymic determination. Hence, they are 
terms which are likely to stand the test of time in our national geologic litera- 
ture. 
GENERAL GEOLOGICAL SECTION OF IOWA ROCKS. 
Eras 
Periods 
Sub-p. 
Series. 
Terranes 
Thick. 
Rocks. 
O 
S ■ 
0 
K 
Quaternarie 
Late 
Recent 
Alluvium 
25 
dlays, sands. 
Mid 
Pleistocene 
Wisconsin 
Peoria 
Iowa 
Sangamon 
Illinois 
Yarmouth 
Kansas 
Alton 
Nebraska 
30 
1 
30 
1 
100 
1 
200 
40 
30 
Till. 
Soils. 
Till. 
Soil. 
Till. 
Soil. 
Till. 
Sands. 
Till. 
Early 
10 
Clays (geest.) 
Tertiarie 
Late 
Pliocene 
Interval 
Unconformity, 
Mid 
Miocene 
Riverside 
50 
Sands. 
Early 
Eocene 
Interval 
Unconformity. 
O 
s 
-<f} 
' Cretaeie 
Mid-__ 
Coloradan 
Niobrara 
Hawarden 
Crill 
Woodbury 
150 
1?5 
100 
150 
Limestones. 
Shales. 
Limestone. 
Shales. 
Dakotan 
Ponca 
Sergeant 
25 
75 
200 
75 
Sandstones. 
Shales. 
Sandstones. 
Shales. 
Nishnabotna 
Dodge 
Early __ _ 
Comanchan 
Interval 
Unconformity, 
Mid 
Missourian 
Atchison 
Forbes 
Platte 
Plattsmouth 
Lawrence 
Stanton 
Parkville 
Thayer 
Bethany 
300 
25 
125 
30 
100 
20 
100 
75 
50 
Shales. 
Limestones. 
Shales. 
limestones. 
Shales. 
Limestones. 
Shales. 
Shales. 
Limestones. 
