Such a process may seem difficult of comprehension. To understand it we must bear in mind some of its conditions. First, the amount of this wrinkling was extremely small relatively to the mass of the earth. In the diagram on page 162 it is greatly exaggerated, yet is seen to be quite insignificant, however gigantic in comparison with microscopic weaklings like ourselves. Secondly, it was probably extremely slow. Beds of solid rock cannot be suddenly bent into great folds without breaking, and the abruptness of some of the folds may be seen from our figure, copied from Rogers (page 162), of some of the foldings of the Appalachian Mountains. Thirdly, the older rocks below the Carboniferous and the Devonian must have been in a softened and plastic state, and so capable of filling up the vacancies left by the bending of the hard crust above. In evidence of this, we have in the Lower Permian immense volcanic ejections--lavas and other molten rocks spewed out to the surface from the softened and molten masses below. Fourthly, the basin of the Atlantic must have been sufficiently strong to resist the immense lateral pressure, so that the yielding was all concentrated on the weaker parts of the crust near the old fractures at the margins of the great continents. In these places also, as we have seen in previous papers, the greatest thickness of deposits had been formed; so that there was great downward pressure, and probably, also, greater softening of the lower part of the crust. Fifthly, as suggested in a previous chapter, the folding of the earth's crust may have resulted from the continued shrinkage of its interior in consequence of cooling, leading after long intervals to collapse of the surface. Astronomers have, however, suggested another cause. The earth bulges at the equator, and is flattened at the poles in consequence of, or in connection with, the swiftness of its rotation; but it has been shown that the rotation of the earth is being very gradually lessened by the attraction of the moon.[T] Pierce has recently brought forward the idea[U] that this diminution of rotation, by causing the crust to subside in the equatorial regions and expand in the polar, might produce the movements observed; and which, according to Lesley, have amounted in the whole course of geological time to about two per cent, of the diameter of our globe. We thus have two causes, either of which seems sufficient to produce the effect. [T] Sir William Thomson, who quotes Adams and Delaunay. [U] "Nature," February, 1871.