The various subterranean changes which have acted upon the coal-fields during the course of unnumbered ages have not only raised or sunk, but frequently dislocated, contorted, ruptured, or broken them up in a most extraordinary manner. In the coal-field near Mons, in Belgium, for instance, a vast lateral pressure has curved the strata again and again, and even folded them four or five times into zigzag bendings, so that on sinking a shaft the same continuous layer of coal is cut through several times. [Illustration: SECTION OF COAL-FIELD SOUTH OF MALMESBURY.] 1. Old red sandstone. 2. Mountain limestone. 3. Millstone grit. 4. Coal seams. 5. Coarse sandstone. 6. New red sandstone. 7. Lias. 8. Inferior oolite. 9. Great oolite. 10. Corn brash and forest marl. Frequently a concave form has been the result of these terrestrial revolutions, and hence coal-fields are often called coal-basins. Thus, in the coal-field south of Malmesbury, the strata appear to dip from the surface, and rise again to it after attaining a certain depth, so that a section of them suggests the idea of a boat or basin. Very commonly one portion of a continuous stratum or series of strata has been broken away from the rest, and has been displaced, either by elevation or depression, or shifting on one side, for various distances. The amount of displacement is sometimes only a few inches, and at other times several hundred fathoms, and the extent may be twenty yards or twenty miles. We may easily conceive the difficulties which these disruptions frequently throw in the way of the miner, who in following what he considers a valuable seam of coal is suddenly stopped by coming in contact with a fault, a trouble, or a slip, as these phenomena are expressively called, and finds the coal shifted several yards above or below, or even completely lost. On the other hand, the miner, thus provokingly stopped in his labours, must not forget that it is perhaps owing to the very shifts he complains of that the outcrop of the coal has occurred at all in his neighbourhood, and that the coal is workable throughout a very large portion of the district in which he is interested.