Coal is a sedimentary rock formed in wide, low-lying equatorial swamps crossed by large rivers and covered by forests of primitive trees. Here, the remains of trees and plants were saved from biodegration and oxidation by mud and water. Coal is usually black in colour but sometimes it occurs as a brownish-black colour. There are four broad ranks or types of coal depending upon its age. Commencing with the youngest and lowest carbon content, these are: lignite, sub-bituminous, bituminous and anthracite. Anthracite is classified as a metamorphic rock because of its subsequent exposure to elevated pressures and temperatures. Bitumen obtained from bituminous coal is a black viscous material generally referred to as 'tar'. Coal is primarily composed of carbon, along with a range of other elements, particularly sulphur. It has been estimated that it takes a layer of peat 6 inches thick to form a vein of coal 1/5th inch thick.

Coal is formed by natural geological processes applying pressure to dead matter. It is principally used for steam/electric generation purposes. Under suitable geological conditions dead matter is formed successively into:

• Peat: Considered to be the precursor of coal.
  
  
• Lignite also known as brown coal: This is the lowest rank of coal. It is soft and still shows traces of plant material. It has a high sulphur content and it burns with a smoky flame. Jet is a compact form of lignite used for ornamental purposes because it can be carved and highly polished.
  
  
• Sub-bituminous coal: The properties of this coal lie between those of lignite and bituminous coal.
  
  
• Bituminous coal: This is a dense black or dark brown coal and it is the most abundant.
  
  
• Anthracite: This is the highest rank of coal and it is a hard glossy-black coal. It consists of up to 95% carbon with a low sulphur content and it burns slowly with a smokeless flame.
  
  
• Graphite: Technically, this is the highest rank of coal but it is difficult to ignite. It is used as pencil lead and in a powdered form as a lubricant.

Other classifications of coal have been identified and these fall in the sub-bituminous and bituminous ranges. They include:

• Flame coal (a form of cannel coal).
  
  
• Gas-flame coal (a form of cannel coal).
  
  
• Gas coal. A by-product of this coal is coke, which is used as a fuel and for metallurgical purposes.
  
  
• Fat coal.
  
  
• Forge coal.
  
  
• Non-baking coal.

Coal was formed during the Carboniferous Period (sometimes called the Coal Age), which lasted from about 360 to 290 million years ago. It was mainly formed from tree ferns, tree horsetails (Calamites) and giant club mosses (Lycopsids) that grew in warm humid swamps. The fossil illustrated below is a Sphenopteris or seed fern that grew during the Carboniferous Period. Seed ferns were ground cover plants and they were among the earliest seed-bearing plants on Earth but they have no descendants.

Seed fern (Sphenopteris), a ground cover plant.

Ferns, horsetails and clubmosses are three families of plants that have survived from the Carboniferous Period. Ferns reproduce by means of minute spores that are usually situated on the undersides of leaves. Horsetails reproduce by means of minute spores that develop in cones at the tip of some stems. Horsetails have hollow, ribbed, roughish, leafless, jointed stems, the joints being covered by toothed sheaths. The joints often bear whorls of similarly ribbed, jointed and leafless branches. Club mosses reproduce by means of minute spores that develop in erect cigar-shaped cones.

It is during this geological period that early conifer trees began to appear on Earth and closely related to these were the Cordaites.

Reconstruction of a carboniferous forest.

It is known that coal miners found fossils of tree ferns, horsetail trees and clubmosses during their work and sometimes they would take them home to show to their families. The fossils illustrated below are typical of those found by miners.