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. [Illustration: SECTION OF PART OF A COAL-FIELD, SHOWING A SUCCESSION OF BURIED TREES AND LAND SURFACE _a_, sandstones. _b_, shales. _c_, coal-seams. _d_, under-clays or soils.] 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. [Illustration: MINE SAFETY CREW] [Illustration: 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.] [Illustration: 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.] [Illustration: 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.] 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 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.