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Volcanic ash that falls into marine settings commonly alters to smectitic deposits known as bentonites, the volcanic origin of which has been recognized for many decades. However, volcanic ash falling into nonmarine coal-forming environments generally alters to kaolinitic claystones called tonsteins, and these beds have only recently been universally accepted as being volcanic in origin. The recognition of tonsteins as altered volcanic ash is based on mineralogy, texture, radiometric age, and field relations.

Tonsteins occur on almost every continent, but are best known from Europe and North America. Their geologic range is coincident with that of coal-forming environments; i.e., from...

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