Coal was 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 a coal seam 1 foot thick needed as much as 7 to 10 feet of peat thickness to commence with.

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

PeatConsidered to be the precursor of coal.

Lignite or brown coalThis 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.

Lignite or brown coal.

Sub-bituminous coalThe properties of this coal lie between those of lignite and bituminous coal.

Bituminous coalThis is a dense black or dark brown coal and it is the most abundant.

Cannel coal (or candle coal)is a type of bituminous coal, which, due to its composition of organic matter and texture, is considered to be oil shale. Being rich in oils, this coal has a waxy lustre and is long burning with a bright yellow flame and little ash.

Bituminous coal

Cannel coal

Steam coal Used to fuel steam engines.

AnthraciteThis is the highest rank of coal. It is hard glossy black and it burns slowly with a smokeless flame. It consists of up to 95% carbon with low sulphur and particulate content.

GraphiteTechnically, 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.

JetA coal-like substance which is the fossilised wood of a tree species similar to the extant Araucaria araucana, known as the Monkey Puzzle tree or Chilean pine, that was formed some 180 million years ago during the Jurassic Period. It is used for ornamental purposes because it can be carved and highly polished.

Jet

Coal was formed during the Carboniferous Period, which was from about 358.9 to about 298.9 million years ago. It was mainly formed from tree-like plants that grew in warm humid swamps:

Seed FernsSphenopteris, includes small plants and trees

Seed TreesMedullosales, up to 35 feet tall

Tree FernsPsaronius, up to 50 feet tall

Horsetail TreesCalamites, up to 65 feet tall

Giant Club MossesLepidodendron, up to 130 feet tall, related to club mosses (Lycopodiaceae)

SigillariaUp to 95 feet tall, related to club mosses (Lycopodiaceae)

StigmariaThe collective name for Lepidodendron and Sigillaria

Sigillaria tree

Carboniferous forest

Giant club moss

Stigmaria root system in Manchester Museum. This fossil was discovered in a quarry at Clayton, Bradford, Yorkshire, in 1886.

During the Carboniferous Period early conifer trees began to appear that were cone-bearing seed plants closely related to seed plants called Cordaites, which are now extinct.

Four families of plants that have survived are:

• Ferns (Filicopsida): These reproduce by means of spores that are usually situated on the undersides of leaves.
• Horsetails (Equisetaceae): These 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. They reproduce by means of minute spores that develop in cones at the tip of some stems.
• Club Mosses (Lycopodiaceae): These reproduce by means of minute spores that develop in cigar-shaped cones.
• Lesser Club Mosses or Spike Mosses (Selaginellaceae): These reproduce by means of minute spores.

Coal miners found plant fossils during their work and sometimes they would take these home to show their families. The two fossils illustrated below are typical of those found by miners.

Above left: Calamite. A genus of extinct tree-like horsetails to which modern horsetails (genus Equisetaceae) are closely related. These plants had distinct nodes, similar to modern horsetails, and their branches and leaves emerged in whorls from these nodes. Their upright stems were woody and connected by an underground runner system. Above right: Sphenopteris or seed fern. These ferns varied in size from small plants to trees and they were among the earliest seed-bearing plants but they have no descendants.