After the formation of the covering of sediment, the decay of the vegetable matter was checked, but a slow change of another kind was brought about by the pressure of the sedimentary deposits and the heat to which the plant remains were subjected. The hydrogen and oxygen which constituted the greater part of the plant substance was driven off and the carbon left behind. This change took place very gradually, through periods so long that we can only guess at their duration, but we know that many beds of coal were formed from layers of vegetable matter that were covered up many thousand years ago. [Illustration: MINE WORKERS THAT NEVER SEE DAYLIGHT Underground stable constructed of concrete and iron, with natural rock roof to avoid danger of fire. Mules are only taken to surface when mines are idle.] The coal first formed and submitted longest to pressure is known as hard coal, or anthracite. It is pure black, or has a bluish metallic luster. Its specific gravity is 1.46; which is about the same as that of hard wood. Anthracite contains from 90 to 94 per cent. of carbon, the remainder being composed of hydrogen, oxygen, and ash. [Illustration: The Mules and their drivers.--An important part of the haulage system. Mules are kept in stables on surface at this mine and driven in every day through slope or drift.] Hard coal may be called the ideal fuel and is especially adapted to domestic heating purposes. It burns without smoke and produces great heat. There is no soot deposit upon the walls of chimneys, and in good stoves or furnaces the small amount of gas given off by it is consumed. Anthracite is the least abundant of all the varieties of coal and is much more costly than the other varieties. For this reason it is not much used in manufacturing. [Illustration: HOW THE SLATE PICKERS WORK Boy slate pickers. Coal slides down the chutes. Boys pick out the slate and rock and throw into chute alongside.] [Illustration: Spiral slate pickers do work of many boys. Coal and rock start together at the top in the small inner spiral. The coal being lighter slides faster, and in going around is carried over the edge into the outer spiral, while the rock continues in the bottom.] The coal formed later is very different in composition and is called bituminous or soft coal. Its name is derived from the fact that it contains a soft substance called bitumen, which oozes out of the coal when heat is applied to it. Soft coal contains from 75 to 85 per cent. of carbon, some traces of sulphur, and a larger percentage of oxygen and hydrogen than anthracite. When soft coal is heated in a closed vessel or retort, the hydrogen and oxygen, in combination with some carbon, are driven off. [Illustration: HOW A COAL MINE LOOKS INSIDE Shaft gate. One of the two cages in the shaft has just brought the men to the surface; the other is at the bottom. Safety gate resting on top of cage covers top of shaft when cage is down, as shown at right.]