Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
World Figure Skating Championships cumulative medal count World Figure Skating Championships cumulative medal count The following is the "cumulative medal count" for countries at the World Figure Skating Championships. It includes countries that no longer exist. Boldface denotes active skaters and highest medal count among all skaters (including these who not included in these tables) per type. Boldface denotes active skaters and highest medal count among all skaters (including these who not included in these tables) per type. Boldface denotes active skaters and highest medal count among all pairs or separate partners (including these who not included in these tables) per type. Italic denotes statistics of performances