Potassium chelation by land plants and the consequent depletion of K+ ion activity in soil waters make K-feldspars unstable in present-day soils. Other factors remaining constant, the absence of land plants in pre-Silurian time would make K-feldspars stable. Dissolution of plagioclase under the action of soil water would be the same in both pre- and post-Silurian time. The presence of abundant fresh K-feldspars and the absence of plagioclase in the Cambrian-Ordovician arenites of Ohio, Wisconsin, Minnesota, Missouri, Texas, Wyoming, and Montana are probably due to the absence of potassium chelating agents in the soils of the time.

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