Thin sections of the Salem limestone from outcrops in the Mississippi River bluffs in southwestern Illinois contain tiny, calcareous spheres which are prominent elements in certain types of limestone. These microorganisms are referable to the genus Calcisphaera established by Williamson. They vary in size, having diameters of 30 to 165 microns. They occur most abundantly in calcisiltites and calcarenites in which microcrystalline, nonskeletal grains are important rock-forming elements. They also occur in calcilutite.

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