People are beginning to ask where fuel will be obtained when the coal-beds are exhausted and the petroleum is all pumped out of the earth. The cold winters will not cease to come regularly, and we shall continue to need fires for many purposes. This is a question which need not trouble us. So long as the sun lasts in the sky and the oceans cover so much of the earth, and so long as there are mountains upon the land, there must be streams with rapids and waterfalls. The power of these streams, which has for ages gone to waste, is now being turned into electricity for purposes of light and heat. We may be sure that long before the mines cease to produce coal and the wells to supply petroleum, there will be something better ready to take their places.

But coal and petroleum are still such important commodities that everyone should know something about the way in which they were made. This earth of ours has had a very long history, much of which has been recorded in the rocks beneath our feet, and the record is more accurate than are many human histories which have been preserved in the printed books.

The story of the earth has been divided into different periods, each marked by the predominance of certain kinds of living things. The Carboniferous period has been so named because at that time the climate and features of the earth in many places favored the growth of dense and heavy vegetation. This vegetation accumulated through the long years, so that it formed thick deposits which gradually changed to beds of coal. It would be wrong, however, to think that all the beds of coal were formed at about the same time. Ever since there have been forests and marshes upon the earth there have been opportunities for the forming of coal-beds. Materials are accumulating even now which will in time be transformed to beds of coal.

We must be equally careful to gain correct ideas of the making of petroleum, for many wrong notions are current. While coal has come from the accumulation of plant remains, petroleum has been derived from sea organisms, chiefly animals. If coal and petroleum are found near each other, the occurrence is accidental and does not mean that the two substances are in any way related.

Our earth is very old, and its surface has gone through many transformations; mountains, plains, and portions of the sea floor have changed places with one another. Wherever there have been marshy lowlands, since plants first began to grow luxuriantly upon the earth, it has been possible for beds of coal to be formed. We all know how rankly plants grow where there is plenty of heat and moisture. Many of us have been in swampy forests and have seen the masses of rotting tree trunks, limbs, and leaves. Now, if we should form a picture in our minds of such a swamp slowly sinking until the water of some lake or ocean had flowed over it and killed the plants, and then washed sand and clay upon the buried forest until it was covered deeply in the earth, we should understand how the coal-beds began. Veins of coal that have been opened by the miners frequently show trunks and stumps of trees, as well as impressions of leaves and ferns. Underneath the coal there is usually a bed of clay, while above sand or sandstone is commonly found.

The oldest coal has been changed the most. It is hard and rather difficult to ignite, but when once on fire it gives more heat and burns longer than other coals. This coal, known as anthracite, is not found extensively in the United States outside of Pennsylvania. Coal which is younger and has been less changed by the heat and pressure brought to bear upon it when it was buried deep in the earth, is known as bituminous. This is the kind of coal which is found in the Mississippi and Ohio valleys, in the Rocky Mountains, and upon the Pacific slope. A still younger coal, which is soft and has a brownish color, is called lignite, and is found mostly in the South and West.

Still another sort of fuel, known as peat, is found in swamps where considerable vegetation is now accumulating, or has accumulated in recent times. Peat is a mass of plant stems, roots, and moss, partly decayed and pressed together. In countries where wood is scarce peat is cut out, dried, and used for fuel.

The larger part of the coal in the eastern United States was formed during the Carboniferous period. That part of our country was then low and swampy; but the West, which is now an elevated area of mountains and plateaus, was at that time largely beneath the ocean.

Then, as the surface of the earth continued to change, the ocean retreated from the Rocky Mountain region, and extensive marshy lowlands with lakes of fresh or brackish water came into existence. There were such marshes in the areas that are now covered by New Mexico, Colorado, Wyoming, Dakota, and Montana. Westward for some distance the land was higher, but in the states of Washington, Oregon, and California there were other marshy lowlands covered with heavy vegetation.

We know from what we have seen of the manner in which wood decays, that in the dry, open air it does not accumulate, but is in great part carried away by the wind. It is only in swamps and shallow bodies of water that the decaying wood can gather in beds. From these facts we have a right to draw conclusions as to the former nature of the surface where there are no coal-beds. There are extensive beds of limestone in the western United States which are of the same age as the coal-beds in the east. As such beds of limestone could have formed only in the ocean, their presence throws a good deal of light upon the geography of those distant times.

Upon the Pacific slope the marshes were not so extensive, nor did they last for so long a period, as those in the East. Nature seems to have confined her strongest efforts at coal-making to the country east of the Rocky Mountains. Perhaps she thought that the people of the West would not need coal if she gave them plenty of gold and silver.

In the Appalachian mountains Nature folded the strata and left them in such a position that the coal could be mined easily. In the Mississippi Valley the beds were left flat, almost in their original position, so that shafts had to be sunk to reach the coal. Upon the Pacific slope Nature seems to have had a large amount of trouble in arranging things satisfactorily. She has made and remade the mountains so many times, and folded and broken the crust of the earth so severely where the swamps stood, that now large portions of the coal beds which once existed have crumbled and been washed away by the streams. The scanty supply of coal which now remains is in most places hard to find and difficult to mine.

The best coal mined near the Pacific comes from Vancouver Island. Large beds of a younger and poorer coal are found southeast of Puget Sound. There are other beds in the Coast ranges of western Oregon, and a few small ones in the Coast ranges of California. The great interior region between the Rocky Mountains and the Coast ranges has very little coal. The people of California have to import large quantities of coal. Some is brought by the railroads from the Rocky Mountain region, but the most comes by ships from various parts of the world, from England, Australia, or British Columbia. The ships bring the coal at low rates and take away grain and lumber.

Coal is almost the only important mineral which Nature has bestowed sparingly upon the Pacific slope. In California, however, she has made amends by storing up large quantities of petroleum. In Pennsylvania and Ohio there is petroleum as well as coal. Oil has also been discovered in the Rocky Mountain region and in Texas.

Petroleum is found flowing from the rocks in the form of springs, either by itself or associated with gases and strong-smelling mineral water. The oil is usually obtained by boring wells, but in southern California there is one mountain range which furnishes large quantities through tunnels which have been run into its side. Petroleum is commonly found in porous sandstones or shales, from one or two hundred to three thousand feet below the surface. It was not made in these rocks, but has soaked into them just as water soaks into a brick. The rocks which produced the oil or petroleum are dark, strong-smelling shales or limestone. Heat a piece of such rock, and you will drive out a little oil.

Examine a piece of the shale from one of the oil districts of California, and you will discover that it is a very peculiar rock, for it is made up almost wholly of minute organisms which once inhabited the ocean. Among the forms which you will find are the silicious skeletons of diatoms, the calcareous skeletons of foraminifera, scales of fish, and, rarely, the whole skeleton of a fish.

Where now there are mountains and valleys dotted with oil derricks, there was once the water of the open ocean. This water was filled, as the water of the ocean is to-day, with an infinite number of living things. As these creatures died, their bodies sank to the bottom, and while the soft parts dissolved, the hard parts or skeletons remained. Through perhaps hundreds of thousands of years, the skeletons continued to accumulate until beds were formed hundreds or even thousands of feet in thickness. The materials of the beds, at first a soft mass like the ooze which the dredger brings up from the bottom of the present ocean, became packed together in a solid mass.

Then disturbances affected this old sea bottom. It was raised, and gravel, clay, and sand from some new shore were washed over the bed of animal remains, burying it deeply. Continued movements of the earth finally folded these rocks, which, as they were, squeezed and broken, became warm. The heat and pressure started chemical action in the decayed animal bodies, and particles of organic matter were driven off in the form of oil and gas. These substances were forced here and there through the fissures in the rocks. Part of the products found a way to the surface and formed springs, while other portions collected to form vast reservoirs in such porous rocks as sandstone. The sulphur and mineral springs which occur in oil regions tell us that this work of oil-making is still going on.

The oil as it comes from the ground is usually brownish or greenish in color, and much thicker than the refined product which we use in our lamps. Some of the crude petroleum is thick and tar-like in appearance, and when long exposed to the air turns to a solid black mass called "asphaltum." This, when softened by heat and mixed with sand, makes a valuable material for street pavement.