Another important type of preservation the are permineralisations. The best known type is the silification. Silicifications are frequently found in areas with active volcanism. Other important permineralisation types are the pyritisation and the so-called coal balls. These latter are carbonate concretions which were formed in peats before these were transformed into coal.
Coal balls are one of the best sources of anatomical information on Carboniferous plants.
Although also Devonian coals are known, especially during the Late Carboniferous thick peats accumulated which were later transformed into coal . During the Carboniferous large parts of Europe and North America were situated in an equatorial position and extensive swamp forests developed in the wetland areas at the continental margins. Temporary sea level changes resulted in flooding of the lowland areas and marine sediments were deposited. Marine bands are intercalated in the mainly terrestrial sequences. The Carboniferous coal floras of Europe and North America are well studied, not at least because of the interests of the mining industry.