1.2 The origins of coal
If you examine a piece of coal, at first sight it appears black and rather homogenous. However, closer inspection generally shows a series of parallel bands up to a few millimetres thick. Most obvious are shiny bands that break into angular pieces if struck. Between them are layers of dull, relatively hard coal and thin weak layers of charcoal-like carbon. Coal splits easily along these weak layers, which crumble to give coal its characteristic dusty black coating.
Microscopic examination of these bands shows that the bright coal represents single fragments of bark or wood, often showing a well-preserved cellular structure. The hard coal is an assemblage of crushed spores, fine wood debris and blobs of resin. The charcoal-like layers comprise charred fragments of bark and wood probably produced by oxygen-starved burning beneath the surface while dead vegetation was accumulating. These features suggest that coal forms from the highly compressed remains of land plants. Further evidence for this is provided by the preservation of individual plant fragments in sediments associated with beds of coal (Figure 1).