Calcareous sediments are defined, but it is shown that sediments of calcareous composition enter into most deposits of all kinds of marine sediments. The fundamental condition controlling accumulation of calcareous sediments is that the sites of deposition are free from entrance of considerable

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quantities of muddy and sandy sediments but such condition does not insure that calcareous sediments will be deposited. Calcareous sediments are precipitated in many ways; some chemical, some physico-chemical, and, most important generally, organic. Organic precipitation is done by both plants and animals, the latter generally readily identified, the former generally indeterminate. Sea bottoms beneath shallow marine waters and protected against entrance of muddy and sandy sediments either because provenances of the sediments are unable to supply such either by reason of being very low or the surface being excellently protected by vegetation, or the sites of deposition being a long way from shore, are the environments that permit the accumulation of widespread deposits of shallow-water marine calcareous sediments. Elevated places on bottoms beneath shallow marine waters may permit calcareous sediments to be deposited in the midst of muddy and sandy sediments. Deep bottoms so far from land that muddy and sandy sediments do not commonly enter are also places of deposition of calcareous sediments.

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