ALTERNATION OP FOSSIL FAUNAS. 
BY CHARLES R. KEYES. 
Several years ago while studying the coal measures of Iowa and the 
neighboring states, it was suggested in a general way that the so-called 
Lower Coal Measures and the so-called Upper Coal Measures eventually 
might prove to be essentially contemporaneous. 
In commenting upon the great erosion plane at the base of the Coal 
Measures it was stated that in the Iowa region at least for a very con- 
siderable period during the Kaskaskia epoch erosive agencies were actively 
at work on the land surface which extended southward about as far as 
the present city of Saint Louis. Shore deposits, sands and clays, were 
laid down immediately beyond the place just mentioned, while farther 
southward marine beds continued to be formed one above another con- 
formably. 4i ,, ; 4]' 
When a new period of depression set in, coal marshes were formed 
along the landward creeping shore-line. The more strictly marine de- 
posits began to slowly extend farther and farther northward resting on 
the older calcareous beds as well as the earlier formed marginal areas 
of sands, clays, and accumulated vegetation. This process with many 
brief interruptions continued until the old shore-line had again gained 
its former place near the present lowa-Minnesota boundary. The coal, 
or marginal, beds were formed at the same time as certain limy layers 
farther outward; and that all formations along any given horizontal 
line (nearly horizontal, .but having a slight inclination to the southwest) 
were deposited contemporaneously. On a sinking coast the marginal 
sediments would have continually the later open sea deposits laid down 
upon them. The covering of the coal-bearing strata by the calcareous 
beds would constantly take place as long as the depression of the shore 
continued. 
The “Lower” Coal Measures are not then a series of beds laid down 
previous to the deposition of the “Upper” Coal Measures. Each particu- 
lar part of the former was deposited at the same time as portions of the 
latter farther seaward; the lines of contemporaneous deposition being 
nearly horizontal, yet having a common though slight seaward tilt. 
As a whole the “Lower” Coal Measures do actually lie beneath the 
“Upper” Coal Measures; but the line of separation is not a line drawn 
parallel, but obliquely to the planes of sedimentation. 
More recently some instructive facts bearing upon the question have 
been brought to light regarding the faunas contained in the Kansas sec- 
tion. Previous results of very similar character were obtained in Iowa 
and Missouri a decade previous, and incidental mention made of them, 
