Fire Clay
Underclay, which consists of siliceous refractory clay rich in hydrous aluminium silicates, is also called fireclay. Just as not all underclays are fireclays, not all fireclays are underclays. Within Carboniferous and other coal bearing strata, fireclay quite commonly comprises many underclays. In Great Britain underclays, which are 1 to 3 metres ( 3 to 9 feet) thick, are major source of commercial fireclay deposits. The alteration of sediments by weathering, plants, and other soil processes comprising underclay resulted in the formation of vast majority of fireclay that comprises underclay.
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