ai6 VOLCANIC ROCKS. f fpontaneous or accidental inflammation of beds of coal. 3. Numerous appearances of this kind are to be obferved in Bohemia, and fome in Scotland and England. Many of the volcanoes in Kamfchatka, and other countries, that throw out only flame and fmoke, are to be confidered as pfeudo-volcanoes. 4. The effedts of thefe volcanoes is very limited and inconfiderable, in comparifon with thofe of true volcanoes. 2. True Volcanoes. 1. Volcanoes have ufually a conical lhape, and are provided fometimes with one, fometimes with feveral mouths or craters. 2. Volcanic mountains in a ftate of adlivity, are thofe that throw out, from time to time, fmoke, vapour,'flame, glowing and melted mafies, allies, and more rarely water. 3. Thefe eruptions alternate in longer or Ihorter periods, and are ufually accompanied with earth quakes and lightning. 4. The different degrees of adlivity of a volca no are well diftinguilhed by the phenomena that occur. The loweft degree of adlivity is when it throws out fmoke, and then it is faid to be in a ftate of reft. This fmoke is compofed of aqueous vapour, muriatic and fulphureous gafes; alfo azote, carbonic acid, and hydrogen gas. There are depo- fited