When a bed of coal is worked out, pillars or rectangular masses of coal are left at intervals as props to support the roof, and protect the colliers. Thus in Fig. 59, representing a section at Wallsend, Newcastle, the galleries which have been excavated are represented by the white spaces _a, b,_ while the adjoining dark portions are parts of the original coal seam left as props, beds of sandy clay or shale constituting the floor of the mine. When the props have been reduced in size, they are pressed down by the weight of overlying rocks (no less than 630 feet thick) upon the shale below, which is thereby squeezed and forced up into the open spaces. Now it might have been expected that, instead of the floor rising up, the ceiling would sink down, and this effect, called a “thrust,” does, in fact, take place where the pavement is more solid than the roof. But it usually happens, in coal-mines, that the roof is composed of hard shale, or occasionally of sandstone, more unyielding than the foundation, which often consists of clay. Even where the argillaceous substrata are hard at first, they soon become softened and reduced to a plastic state when exposed to the contact of air and water in the floor of a mine. Fig. 59: Section of carboniferous strata at Wallsend showing ‘creeps’. The first symptom of a “creep,” says Mr. Buddle, is a slight curvature at the bottom of each gallery, as at _a_, Fig. 59: then the pavement, continuing to rise, begins to open with a longitudinal crack, as at _b_; then the points of the fractured ridge reach the roof, as at _c_; and, lastly, the upraised beds close up the whole gallery, and the broken portions of the ridge are reunited and flattened at the top, exhibiting the flexure seen at _d._ Meanwhile the coal in the props has become crushed and cracked by pressure. It is also found that below the creeps _a, b, c, d,_ an inferior stratum, called the “metal coal,” which is 3 feet thick, has been fractured at the points _e, f, g, h,_ and has risen, so as to prove that the upward movement, caused by the working out of the “main coal,” has been propagated through a thickness of 54 feet of argillaceous beds, which intervene between the two coal-seams. This same displacement has also been traced downward more than 150 feet below the metal coal, but it grows continually less and less until it becomes imperceptible.