352 
is very different from that of southeastern Mis- 
souri, and much confusion attended the earlier 
attempts to refer this formation to its proper 
position in the series. This confusion was evi- 
dently due, in large part, to the failure of 
earlier workers to recognize the disconform- 
ities at the base and at the top of the forma- 
tion. The apparent tendency of the Spergen 
to grade laterally into the Warsaw or the Saint 
Louis has resulted entirely from these relation- 
ships. In addition, the Spergen is very variable 
in lithologic character in this region, due in 
part to original conditions of sedimentation, 
and in part to differences in the degree of 
dolomitization. It is not uncommon to find 
a cross+bedded, crinoidal limestone passing 
laterally within a short distance through im- 
perfectly dolomitized limestone into massive, 
brown dolomite, and this again into a brown- 
ish arenaceous dolomite, which may in turn 
give way to a fine-grained, bluish sandstone. 
Such rapid changes clearly indicate near-shore 
conditions during deposition. This is also 
suggested by the limited extent of the forma- 
tion in Iowa. and by its rapid thinning to the 
northwest. Its thickness in this region varies 
from 0 to 35 feet. 
The Saint Louis limestone of Iowa also 
shows marked evidence of shallow conditions 
during deposition, although it has a much more 
widespread distribution than the Spergen. It 
consists of two distinct subdivisions separated 
from one another by a disconformity. For 
convenience these may be designated as the 
Lower Saint Louis and the Upper Saint Louis. 
The Lower Saint Louis is by far the most ex- 
tensive of the two members. This extends far 
to the northward, overlapping all the earlier 
formations of the Mississippian except the 
Kinderhook, upon which it rests in Humboldt 
county. It consists for the most part of mas- 
sive beds of compact, dolomitic limestone, but 
frequently these are found to grade laterally 
into gray, non-dolomitic limestone within short 
distances. At most localities, the lower beds 
are arenaceous. Ripple marks and cross-bed- 
ding may appear locally at any horizon. In 
southeastern Iowa, mound-like reefs of lime- 
stone with undisturbed layers lapping up on 
SCIENCE 
[N. 8. Vou. XLVIII. No. 1240 
their flanks are occasionally found in the for- 
mation. These were evidently formed by 
wave action during deposition. The thickness 
of this division is about thirty feet. The 
Upper Saint Louis consists for the most part 
of light gray compact limestone which is lo- 
cally dolomitized either wholly or in part, and 
shows the same evidence of shallow water de- 
position as the Lower. Locally this division 
passes laterally into sandstone in part. The 
Upper Saint Louis seldom exceeds twenty- 
five feet in thickness. 
The writer has observed further evidence 
of the relation of the extensive dolomitization 
to the shallow water zone in the Cedar Valley 
limestone, of Upper Devonian age, in Iowa. 
In Johnson county, which is located a short 
distance south of the east-central portion of 
the state, this formation has an exposed thick- 
ness of approximately one hundred feet and 
consists of fairly pure, gray fossiliferous lime- 
stone almost entirely devoid of dolomite. But 
in Mitchell, Howard, Winneshiek and other 
counties in the northern portion of the state, 
the Cedar Valley is made up of interbedded 
limestone and dolomite, and bears evidence of 
having been deposited in shallow seas. The 
beds are impure, shaly partings are common 
between the layers, and evidences of contem- 
poraneous erosion are frequently encountered. 
The suggestion is ventured that careful 
study of other Paleozoic limestones will dis- 
close similar evidence of more extensive dolo- 
mitization in their shallow water facies. 
Franois M. Van Tuyn 
CotoraDo ScHooL oF MINES 
SCIENCE 
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