The record of these long-ago events is found when we sink a shaft through typical coal-bearing strata. We pass through not one, but several, layers of coal, which may vary in thickness from a fraction of an inch to a hundred feet or more, and are separated by generally much thicker layers of sandstone or shale (solidified clay). The layers of coal are known as “coal-beds.” Unless a coal-bed is at least two feet thick it is hardly worth working, and, ordinarily, the thickness of a bed does not exceed eight or ten feet. The shale or sandstone above a bed is very commonly found to contain the remains or the impressions of the ancient plants from which the coal was formed. A study of these remains and casts has made it possible to classify hundreds of species of plants now extinct. Fragments of plants are also sometimes found in the coal itself, and thin slices of coal frequently show a vegetable structure under the microscope. Finally, to furnish conclusive proof of the vegetable origin of coal, we find under the coal-bed a layer known as the “underclay,” which is a fossil soil filled with the roots and rootlets of the coal-producing plants. Different conditions of formation, and also, probably, differences in the character of the original vegetation, have resulted in the production of different kinds of coal. The most important heat-producing constituent in coal is the elementary substance called “carbon,” and the simplest classification of solid fuels depends upon the percentages of fixed (non-volatile) carbon they contain, the average percentages running as follows: Wood, 50%; peat, 55%; lignite, 73%; bituminous coal, 84%; anthracite coal, 93%. When fuel is burned the greater part of it unites chemically with the oxygen of the air to form certain invisible gases--especially carbon dioxide and water-vapor--and only the ash remains. [Illustration: From “Geology, Physical and Historical,” by H. P. Cleland. American Book Co., N. Y. Section of coal-bearing strata in Pennsylvania, showing relative amount of coal and barren rock in a rich field] In the popular mind coal is classified as hard or soft, while hard coal is further classified according to the size of the lumps. For both scientific and industrial purposes more elaborate classifications are necessary, and several have been used or proposed. [Illustration: COAL FIELDS OF THE UNITED STATES] _Kinds of Coal_ The United States Geological Survey classifies coals, first of all, according to “rank,” depending upon both chemical and physical characteristics. Anthracite, which contains the largest percentage of carbon, ranks highest, and lignite, with the smallest percentage of carbon, lowest. Coals of the same rank are said to be of high or low “grade,” according to whether they contain a relatively small or large percentage, respectively, of ash and sulphur. The ranks recognized by the Survey are: Anthracite, semi-anthracite, semi-bituminous, bituminous, sub-bituminous, and lignite.