Such, in brief, is the history of the earth as the rocks have told it to us. Without their help we could know but little of the story. Through all the periods of time and all the ages, they were being formed, layer upon layer, of sand and silt, of mud and pebbles, hardening with the passing of the centuries. But while they were still soft they received impressions of the feet of birds and of beasts, they were marked by the waves and were cracked in the fierce heat of the sun, and their surfaces were pitted by the rain-drops of passing showers. Shells, corals, and sponges were imbedded in them; the skeletons of fishes and the bones of animals that walked or crept upon the land or flew in the air were covered over by them; they caught and held the drooping fern, the falling leaf and twig and nut; they closed around the body of the tree itself and buried it from sight; and as the soil hardened into rock, bone and shell, leaf and stem, hardened with it and became part of it. To-day we find these fossil remains, sometimes near the surface of the earth, sometimes hundreds or thousands of feet below it. We uncover them from the soil, we break them from the rock, we blast them out in the quarries, we dig them from the mines of coal and ore. It is by them and by the structure of the rock which contains them that we read the history of the earth, a history covering so long a period of time from the beginning of the stratification of the rocks to the age when man appeared upon the globe that no one has yet dared to reckon the millions upon millions of years which intervened, and give the result of his computation to the world as true. [Illustration: COLUMNAR SECTION OF THE EARTH’S CRUST.] CHAPTER II. THE COMPOSITION OF COAL. The first question that would naturally be asked concerning the subject with which we are dealing is, What is coal? In reply it may be said that it is a mineral. It is black or brown in color, solid, heavy, and amorphous. The specific gravity of the average Pennsylvania anthracite is about 1.6, and of the bituminous coal about 1.4. There are four varieties of mineral coal, namely: anthracite, bituminous, lignite or brown coal, and cannel coal. To this list it would not be improper to add peat, since it partakes of most of the characteristics of mineral coal, and would doubtless develop into such coal if the process of transformation were allowed to continue undisturbed. The principal element contained in each of these different kinds of coal is carbon. An analysis of an average piece of Pennsylvania anthracite would show the following chemical composition:―― Fixed carbon 86.4 Ash 6.2 Water 3.7 Volatile matter 3.1 Sulphur .6 ――――― Total 100 The composition of the bituminous coals of Pennsylvania, as represented by the gas coal of Westmoreland County, is shown by analysis to be as follows:――