beds of fire-clay, shale, and sandstone, and sometimes limestone (see Fig. 9). A bed of fire-clay underlies every seam of coal, and a bed of rules. Sometimes the fire-clay has a substitute, in the shape of a bed of nearly pure silica, called ganister, which is very hard, but this is very rare; and sometimes, though seldom, a sandstone takes the place of the shale roof. The seams of coal in Great Britain are remarkable for their regular thickness and the wonderful manner in which the charac teristics of each particular seam are often maintained for many miles. A seam of coal, say 4 feet thick, may sometimes be traced for 5 or 6 miles without a variation in thickness exceeding 6 inches, whilst the variations in quality will be hardly perceptible. A seam of coal sometimes maintains its distinctive qualities for a distance of more than 40 miles.1 The natural tendency is towards variations, and coals thicken or thin, or split up into several seams (see Fig. 23) or are brought FIG. 23.-Three seams of coal FIG. 24. Sandstone merging into together into one thick seam (as in the Staffordshire thick coal, which is 30 feet thick in one great bed, with only thin dirt partings, and in a distance of about 10 miles intercalations of sandstone and shale are included, having an aggregate thickness of FIGS. 25, 26.-Lepidodendron selaginoidis (transverse sections). Sketches on line a b. 500 feet); they vary from clean to dirty and from anthracitic to bituminous. In the same way the shales gradually merge into sandstones, and vice versa (see Fig. 24). Soft fire-clays change into hard or indurated fire-clays. In one part of a coal-field two well-known seams may be separated by say 240 yards of strata, and within say 16 miles the intervening strata may be reduced to 180 yards. These changes are what must always be expected; it is only the regularity that at all surprises the geological student. 1 Cannel coal is an exception to all the above statements. In the Coal Measures the stumps of trees are often found erect, as if in the place where they had grown. The origin of coal can be shown by grinding sections of it till FIG. 28.--Vertical section of Derbyshire Coal-field between Chesterfield and Nottingham. they are transparent, then, if a bright light is focussed on these sections and a double convex lens placed in front, the structure of the specimen may be displayed upon a white screen; and it is thus seen to bear the characteristics of wood and vegetable growth (see Figs. 25 and 26). The following are some sections of coal-fields: Fig. 27, section across the Midland Coal-field; Fig. 28, vertical section of the Derbyshire Coal-field between Chesterfield and Nottingham. The above sections were levelled and measured by the author for the Royal Commission in 1867-68. The following list of minerals includes most of those known commercially to miners. Those printed in darker type are the most important as regards commercial value. Nearly all of them are got in British mines, though often only in very small proportions. But platinum, mercury, and the more valuable precious stones are not found in British mines. Some of the minerals and metals whose names are given below and in the following pages occur in a great variety of forms and chemical combinations, to each of which mineralogists have given a name. There are many hundreds of these, for the study of which the reader is referred to books on mineralogy and metallurgy. Gypsum. (E.) Graphite. (E.) Iron ore. (E) Iridium ore. Jet. (E.) Kaolin. Lead ore. Lithium ore. Lignite. (E.) TABLE II.—LIST OF FIFTY-EIGHT MINERALS. E denotes that they are found in Great Britain. Apatite (for phosphorus). Gold. (E.) Alum clay. (E.) Alum shale. (E.) Limestone. (E.) Petroleum. (E.) (Only slight indications in Potassium ore. Precious stones. Silver ore. (E.) Slates. (E.) Sodium nitrate. (E.) Salt. (E.) (E.) Stone. (E.) Coa!. (E.) Clays (Fire-clay). Cobalt ore. Mica. (E.) Of the above minerals many are found deposit or disseminated in stratified rocks. shown in the following table : Tellurium ore. Tin ore. (E.) |