52 CURVED STRATA. [Oh. V. 



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. 



The first symptom of a " creep," says Mr. Buddie, is a slight curvature 

 at the bottom of each gallery, as at a, fig. 66 ; then the pavement con- 

 tinuing 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 reimited 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, 6, c, d, an inferior 

 stratum, called the " metal coal," which is 3 feet thick, has been fractured 

 at the points e, /, g, k, 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 downwards more than 150 feet below the metal coal, but it 

 grows continually less and less until it becomes imperceptible. 



No part of the process above described is more deserving of our no- 

 tice than the slowness with which the change in the arrangement of the 

 beds is brought about. Days, months, or even years, will sometimes 

 elapse between the first bending of the pavement and the time of its 

 reaching the roof. Where the movement has been most rapid, the curv- 

 ature of the beds is most regular, and the reunion of the fractured ends 

 most complete ; whereas the signs of displacement or violence are great- 

 est in those creeps which have required months or years for their entire 

 accomplishment. Hence we may conclude that similar changes may 

 have been wrought on a larger scale in the earth's crust by partial and 

 gradual subsidences, especially where the ground has been undermined 

 throughout long periods, of time ; and we must be on our guard against 

 inferring sudden violence, simply because the distortion of the beds is 

 excessive. 



Between the layers of shale, accompanying coal, we sometimes see 

 the leaves of fossil ferns spread out as regularly as dried plants between 

 sheets of paper in the herbarium of a botanist. These fern-leaves, or 

 fronds, must have rested horizontally on soft mud, when first deposited. 

 If, therefore, they and the layers of shale are now inclined, or standing 

 on end, it is obviously the effect of subsequent derangement. The proof 

 becomes, if possible, still more striking when these strata, including 



