Sec. 19. The next point of importance in these beds is the curvature, to which, as well as to fracture, they seem to have been subjected. This curvature is not to be confounded with that rippling or undulating character of every portion of the slaty crystalline rocks above described. I am now speaking of all kinds of rocks indifferently;--not of their appearance in small pieces, but of their great contours in masses, thousands of feet thick. And it is almost universally true of these masses that they do not merely lie in flat superposition one over another, as the books in Fig. 8; but they lie in _waves_, more or less vast and sweeping according to the scale of the country, as in Fig. 15, where the distance from one side of the figure to the other is supposed to be four or five leagues. Sec. 20. Now, observe, if the precipices which we have just been describing had been broken when their substance was in a hard state, there appears no reason why any connexion should be apparent between the energy of _undulation_ and these _broken_ rocks. If the continuous waves were caused by convulsive movements of the earth's surface while its substance was pliable, and were left in repose for so long a period as to become perfectly hard before they were broken into cliffs, there seems no reason why the second series of shocks should so closely have confined itself to the locality which had suffered the first, that the most abrupt precipices should always be associated with the wildest waves. We might have expected that sometimes we should have had noble cliffs raised where the waves had been slight; and sometimes low and slight fractures where the waves had been violent. But this is not so. The contortions and fractures bear always such relation to each other as appears positively to imply contemporaneous formation. Through all the lowland districts of the world the average contour of the waves of rock is somewhat as represented in Fig. 16 _a_, and the little cliffs or hills formed at the edges of the beds (whether by fracture, or, as oftener happens in such countries, by gradual washing away under the surge of ancient seas) are no higher, in proportion to the extent of surface, than the little steps seen in the centre of the figure. Such is the nature, and such the scale, of the ranges of hills which form our own downs and wolds, and the French coteaux beside their winding rivers. But as we approach the hill countries, the undulation becomes more marked, and the crags more bold; so that almost any portion of such mountain ranges as the Jura or the Vosges will present itself under conditions such as those at _b_, the precipices at the edges being bolder in exact proportion to the violence of wave. And, finally, in the central and noblest chains the undulation becomes literally contortion; the beds occur in such positions as those at _c_, and the precipices are bold and terrific in exact proportion to this exaggerated and tremendous contortion.