Coal is largely formed from vast masses of vegetable matter deposited through the luxuriant growth of plants in former epochs of the earth's history. In the varieties of coal in common use the combined effects of pressure, heat and chemical action upon the substance have left few traces of its vegetable origin; but in the sandstones, clays and shales accompanying the coal the plants to which it principally owes its origin are presented in a fossil state in great profusion and frequently with their structure so distinctly retained, although replaced by mineral substances, as to enable the microscopist to determine their botanical affinities with existing species. Trees of considerable magnitude have also been brought to light.

The animal remains found in the coal-measures indicate that some of the rocks have been deposited in fresh water, probably in lakes, while others are obviously of estuarine origin, or have been deposited at the mouths of rivers alternately occupied by fresh and salt water. The great system of strata in which coal is chiefly found is known as the carboniferous.

There are many varieties of coal, varying considerably in their composition, as anthracite, nearly pure carbon, and burning with little flame, much used for furnaces d, under-clays or soils.

Section of Part of a Coal-field, Showing.

a Succession of

Buried Trees and b, shales.

Land Surface a, sandstones.

c, coal-seams.

Mine Safety Crew.

Mine Rescue Work.

Upper view, Bureau of Mines Rescue Crew in safety helmets, ready to enter a gas-filled mine. Lower view, resuscitating a victim overcome by gas by means of the oxygen reviving apparatus.

Briquetting Machine.

Enormous quantities of coal are lost at the mines in coal dust. By adding a binding material, such as pitch, and pressing the mixture into briquettes or small bricks, an excellent fuel is made.

Mine Rescue Work.

The mine rescue crew is using the canary-bird test for poisonous gas. The bird succumbs to gas earlier than a man and thus indicates a dangerous condition of the atmosphere. The canary is revived by oxygen and the crew puts on safety helmets before proceeding.

and malt kilns; bituminous, a softer and more free-burning variety; and cannel or "gas-coal," which burns readily like a candle, and is much used in gasmaking. The terms semi-anthracite, semi-bituminous, coking coal, splint coal, etc., are also applied according to peculiarities.

All varieties agree in containing from 60 to over 90 per cent of carbon, the other elements being chiefly oxygen and hydrogen, and frequently a small portion of nitrogen. Lignite or brown coal may contain only 50 per cent of carbon. For manufacturing purposes coals are generally considered to consist of two parts, the volatile or bituminous portion, which yields the gas used for lighting, and the substance, comparatively fixed, usually known as coke, which is obtained by heating the coals in ovens or other close arrangements.

About 260,000,000 tons of coal are annually mined in Britain, the value being over $300,000,000. Large quantities are exported. The British coal-fields, though comparatively, extensive (covering about 9,000 square miles), are far surpassed by those of several other countries, as the United States and China, the former having coal-fields estimated to cover about 451,000 square miles; the latter over 200,000 square miles. Britain no longer mines the largest quantity, having been far surpassed by the United States. Other countries in which coal is worked are Belgium, France, Germany, Russia, India, New South Wales and Canada. China has hitherto mined only on a small scale.

The annual production of anthracite coal in Pennsylvania is more than 86,000,000 tons of 2,240 pounds, valued at the mines at $198,000,000. In 1910 there were produced of bituminous coal 388,222,868 tons, valued at $463,654,776; amount of coke manufactured, 37,000,000 tons. This was distributed widely over the country, the greatest producers, after Pennsylvania, being Illinois, West Virginia, Ohio, Alabama and Colorado.

Recently a very large output of coal has been discovered in Alaska, the value of which is as yet undetermined, though it is believed to hold a vast quantity of coal. The value of the western coal-fields also is far from known, and since 1906 very extensive tracts of coal-bearing lands have been withdrawn from settlement, principally in Wyoming, Montana, Colorado, Utah and New Mexico, their beds being largely of lignite. These cover about 50,000,000 acies, and, with those of Alaska, are held by the government as national assets. The mines of Alaska are claimed to be exceedingly rich, both in bituminous and anthracite coal, the beds examined being estimated to contain 15,000,000,000 tons, while there are large districts unexamined. They have not yet been worked, the government keeping them back for public ownership.