Fig. 30.
that the lower single fissure F E widens into a triangular gap, and consequently E C spreads to E C', and E D to E D'. In this case the included triangular mass of beds between D and C must fall down into the open fissure F E. Moreover, as for every inch that the parts A B rise and that E F widens, the included mass D C will settle down into it, the corners at E will be worn and ground down by the friction, so that they will very shortly be ground off to the lines h g, which will have the effect of still further widening the fissure, and consequently of admitting the wedge-shaped piece D C to sink further down into it.
Suppose now that the expansive and elevatory force ceases to act, and the upraised beds begin to settle down again towards a horizontal or nearly horizontal position, it is quite possible for the lower part of the fissure E F to close again, and to close in some cases so accurately and perfectly, that if the beds are at all soft and cohesive, scarcely a trace of it may be left after some period of compression. That part of the fissure, however, into which the wedge has sunk will, of course, not be able to close again, but it will strive to do so, and great force of compression will consequently be exerted on the sides of the wedge, and on the beds which come against them. The effect of this compression is shown in Fig. 26 in the destruction of the lamination in the coal marked B B. As, however, the beds sink into a very low arch, the downward pressure on the summit of the arch is made more and more of a horizontal thrust on its sides or abutments, the parts A and B on the sides of Figs. 29 and 30. When the arch becomes very low, the lateral pressure hereabouts must be enormous; and it is important to remark,[1] that we have here a real cause for bonâ fide lateral pressure, and consequent lateral shifting, and even of the sliding of one bed over the other, so that during the subsidence greater dislocations and more crumplings and contortions may take place at the outer ends of the arched beds, or at some distance outside of them, than took place in any part of the beds during the action of the elevating force.
Fig. 27 is an admirable instance of the arched condition of the beds, of the compressed central parts B B, and of the cracks and fissures that are likely to be caused on the sides of the arch as the
- ↑ This was remarked to me by Mr. Hopkins, in a conversation I once had with him on this point in the Museum of Practical Geology.
