INTRODUCTION. 
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In many places the line of separation between this and the succeed¬ 
ing formation is strongly defined by a shaly bed, containing numerous 
siliceous concretions or geodes. To the Geode bed, and in its absence, 
to the Keokuk limestone, succeeds a mass of shaly limestone with more 
compact layers, the whole becoming sometimes altogether calcareous. 
It is marked by the presence of peculiar Crinoidea and Brachiopoda; 
but its most characteristic fossil is the peculiar fenestelloid bryozoan 
known as Archimedes, the spiral axes of which are often very abundant 
upon the surfaces of thin calcareous beds near Warsaw in Illinois. 
This limestone is more distinctly marked at that place than any other 
known locality, and hence has received its name. Though clearly 
distinct in position and fauna, it has many affinities with the succeed¬ 
ing formation. 
The Warsaw limestone is followed by a*brecciated limestone, which 
is associated with yellow magnesian beds, and sometimes an arena¬ 
ceous bed in its lower division. This formation, designated as the 
St. Louis limestone by Professor Swallow in his Geological Report of 
Missouri, varies from a brecciated mass to a regularly stratified gray 
limestone, with some important beds of light gray subcrystalline lime¬ 
stone. The rock is well marked by its peculiar organisms, the beautiful 
Echinocidarus (Palcechinus multipora of Owen and Norwood), being one of 
its most conspicuous forms. Numerous Bryozoa cover the surfaces of 
some of the layers, which are separated by shaly partings ; while the 
Lithostrotion mamillare, the most conspicuous coral of all the limestones 
of the Mississippi valley, marks this horizon. 
The St. Louis limestone is limited above by a ferruginous sandstone, 
a mass of nearly two hundred feet thick. This sandstone marks, over a 
considerable area, the cessation of calcareous accumulations, and is a 
precursory influx of the arenaceous material which, in. the sandstones 
of the Coal measures, followed the conclusion of the calcareous series. 
This formation occurs in Missouri and in Illinois, at numerous locali¬ 
ties, but thus far little is known of its organic contents. 
