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COAL.

generally in the order of succession in rocks deposited under the sea. In North and SouthWales, in Shropshire, Derbyshire, Yorkshire, Lancashire, Northumberland, and Durham,the coal measures rest directly upon the mountain limestone; but in South Staffordshirethat limestone is wanting, and they rest on the limestone of the lower transition series, whilstin Scotland the old red sandstone forms their foundation.

The regular coal formation has never been found at great elevations, and it generallyoccupies the valleys, near mountain ranges. So far as geological knowledge of the differentcountries on the globe extends, coal is principally found between the latitudes 35° and t>5°.

There has been much speculation on the probable causes of the formation of coal. Allgeologists are, however, now agreed that it is the product of decomposed vegetation ; butopinions differ respecting the nature of the plants from which coal was formed, the processesand changes that, the vegetable substance has undergone during its conversion into mineralcoal, the source whence the carbon of which it is principally composed was derived, and thecondition of the vegetation, as to growth or accumulation, at the time of conversion. Theimpressions and the remains of vegetables in the coal itself, and in the accompanying strata,are sufficient to point out the source whence it was formed; but if any doubt existed as tothe origin of coal, it would be removed by the evidence of the gradual change of wood intomineral fuel, which is afforded in the half converted kind called wood-coal. That kind ofcoal is deposited in beds manv feet in thickness at Bovey Heathtieid, near Exeter, and it isstill more completely developed near Cologne, where it extends over a space of several leagues,in beds fifty feet thick. It rests on clay, and is covered by beds of gravel, being evidentlya more recent formation than that of common coal. In wood-coal the trunks and roots oftrees only partially mineralized are seen in the upper parts of the beds, and lower down thevegetable matter is consolidated, and converted into a substance closely approaching hardmineral coal. Nuts, now indigenous in China and India, and a fragrant resinous substance,called retinasphaltum, have been found in it.

The evidence of very luxuriant vegetation at the period of the coal formation shows thatthe condition of the earth and of the atmosphere must have been peculiarly favourable tovegetable life. It has been conjectured that at that time the atmosphere contained a greaterproportion of carbonic acid than at present. Such a condition would be highly conduciveto the growth of plants, as the carbon that is secreted during the process of vegetation isalmost entirely derived from the atmosphere. Carbonic acid, which consists of carbon andoxygen united in the proportion of 27 to 73, forms an important constituent of limestone,and such vast quantities of carbon are stored up in that form in mountain masses, that itis estimated that the calcareous ranges of the Jura and of the Alps contain sufficient to forma bed of pure solid carbon, 1000 feet thick, that would cover the whole surface of thosemountains.

After the formation of the beds of coal, the earth must have been subjected to violentconvulsions, which disturbed the strata, and frequently altered their original positions.These “ faults,” as they are termed, have been caused by internal action, which has forceda quantity of igneous matter through the strata, extending like a wall sometimes for severalmiles. The effect of these irruptions has invariably been to raise the strata on one side