This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
Oil is made by animals and plants. The flesh of all animals contain fat that is easily melted into oils. Mutton fat or tallow is so waxy that it hardens and makes good candles. Whales have a great blanket or thick layer of fat that men used to melt and burn in lamps. Codfish give a very pure, rich oil that is valuable as a food-medicine for building up sick people. Among plants the olive is best known for its oil, but peanuts, other nuts, cotton seed, castor beans, and many other fruits and seeds are rich in oil. When men first found petroleum far down in the earth, they called it mineral, rock or coal oil. Now we know that there is no such thing as mineral oil. We know that coal and coal oil are both vegetable products. The plants that were pressed into coal beds contained oil, probably in the fruits or seeds. Under pressure, the oil escaped from the woody cells, and collected into pockets or wells in the earth in enormous quantities. Much of the oil in the earth has been vaporized, or turned into gas. This accounts for natural gas.