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
Coal power in the United States 13.1% was emitted by coal-fired industrial and mixed-use commercial boilers, and 0.3% by coal-fired residential boilers, bringing the total U.S. mercury pollution due to coal combustion to 46% of the U.S. man-made mercury sources. In contrast, China's coal-fired power plants emitted an estimated 200 ± 90 tons of mercury in 1999, which was about 38% of Chinese human-generated mercury emissions (45% being emitted from non-ferrous metals smelting). Mercury in emissions from power plants can be reduced by the use of activated carbon. In 2007 an advertising campaign was launched to improve public opinion on coal power titled "America's Power". This