Rates of deposition of marine sediments are shown to be highly variable. Slow rates of deposition permit long contact of the sediments with sea water and with bottom dwelling organisms. These organisms rather thoroughly search the slowly deposited sediments for contained nutrient matter, macerate them in their chewing organisms, pass them through their digestive systems, and more or less completely destroy all shells and tests. The sediments ultimately become excremental in character in which perfect shells may be expected to be uncommon. The more abundant the organisms on bottoms of slow deposition, the less are the possibilities of the sediments containing other than fragments of shells. Slow deposition also leads to the formation of certain minerals of which glauconite is an example. Rapid deposition, on the other hand, swiftly removes sediments from contact with the sea water and the destructive work of bottom dwelling organisms. These sediments contain little excremental matter; they may contain many perfect shells and most of any organic matter deposited with them.

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