. Elements of geology : a text-book for colleges and for the general reader. Geology. ROCK-SYSTEM OR COAL-MEASURES. 347 measures, it is evident that but a small proportion consists of coal. The coal-measures consist, in fact, of thick strata of sandstone, shales, and limestone, like other formations; but in addition to these are interstratined thin seams of coal and beds of iron-ore. Even in the richest coal-measures, the pro- portion of coal to rock is not more than as 1 to 50, and the proportion of iron is still much smaller. In some coal-fields, as, for example, in the Appalachian, mechan-
. Elements of geology : a text-book for colleges and for the general reader. Geology. ROCK-SYSTEM OR COAL-MEASURES. 347 measures, it is evident that but a small proportion consists of coal. The coal-measures consist, in fact, of thick strata of sandstone, shales, and limestone, like other formations; but in addition to these are interstratined thin seams of coal and beds of iron-ore. Even in the richest coal-measures, the pro- portion of coal to rock is not more than as 1 to 50, and the proportion of iron is still much smaller. In some coal-fields, as, for example, in the Appalachian, mechan- ical sediments, shales, and sandstones, predominate; in others, as in the Western coal-fields, organic sediments or limestone predominate. The five kinds of strata mentioned are repeated in the same coal-basin very many times—perhaps 100 or more, as in the accompanying section ; but, in com- paring one coal-field with another,.or in the same coal- field, in comparing one portion of the series with an- other, there is no regular order of sticcession discovera- ble. Except that immediately in contact with the seam, and beneath it, there is nearly always a thin stratum of fine fire-clay. This constant attendant of a coal-seam is called the under-day. Again, immediate- ly above, and therefore forming the roof of the opened seam, there is frequently, though not so constantly, a shale which, being impregnated with carbonaceous matter, is called the black shale or black slate. These accompaniments are, however, usually too thin to ap- pear on sections. In different portions, however, of the same coal-field geological horizon, we are apt to find the same Fig. 450. — Ideal Section, showing Alternation of Different Kinds of Strata: Ss, Sandstone ; Sh, Shale ; I, Lime- stone ; i. Iron, and c, Coal. at the same This is the necessary result of the continuity of the strata over the whole basin. If we represent coal-basins, with their five different kinds of strata, by reams of va
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
