Finally great depth of burial, long time and higher temperature may metamorphose the lignite into soft coal or bituminous coal and in extreme cases, to hard coal or anthracite.

• 褐煤经长时间进一步的深埋以及较高的温度的作用，便可变成软煤或烟煤，再进一步变化，便成了硬煤或无烟煤。