Land plants have the power, under the influence of light, of decomposing the carbon dioxide (carbonic-acid gas) of the air and fixing the carbon in their tissues, the oxygen being liberated and rendered available for animal respiration. Carbon is thus concentrated, and when plant remains accumulate and are preserved beneath water in swamps, a slow change takes place and peat is formed. The essential conditions for the accumulation of vegetable matter have been present on the earth ever since a land flora existed, and coal-beds occur at many different horizons. The earliest date at which land plants seem to have been sufficiently abundant to furnish material for coal-beds was the Carboniferous period. Although a similar flora existed during the preceding period, the Devonian, no coal-beds of workable thickness are known in the rocks of that age. Since the Carboniferous period coal has been found at many horizons in the sedimentary rocks, and peat is being accumulated at the present day. [Illustration: FIG. 34.--Map showing the distribution of coal in North America.] The coal-fields of North America are more extensive than those of any other continent, excepting, perhaps, the at present but little known coal-bearing formations of Asia, and are distributed in temperate latitudes, from tide-water on the Atlantic to tide-water on the Pacific coasts, where the greatest commercial and intellectual development has been reached. Coal of Carboniferous age occurs in large and valuable deposits in Nova Scotia and New Brunswick; there is a small area of graphitic anthracite, not now utilized, in Rhode Island; but the great fields are in Pennsylvania and the States southward to central Alabama, and westward to beyond the Mississippi. A detached coal-basin containing some 6,700 square miles, but a small part of which is productive, however, occurs in the central part of southern Michigan. Small coal-fields in Virginia and North Carolina, the first to be worked in America, are of Jura-Trias age and form part of the Newark system. Extensive fields of valuable coal of Mesozoic age, principally in the Laramie system, occur in New Mexico, Colorado, Wyoming, Montana, and still farther north along the same great belt in Canada. Another highly valuable field of Mesozoic coal is now being extensively worked on Vancouver Island. The coals of the west side of the Pacific mountains, largely lignites, but in many instances of high grade and serviceable for steam coal, are mostly of Cenozoic age (Tertiary) and occur in California, Oregon, Washington, and Alaska. The distribution of the various coal-fields is indicated on the above map, and space will not be taken in describing their geographical relations.