occurrence in stratified hills, and peculiarly important as giving rise to the serrated structure, rendered classical by the Spaniards in their universal term for mountain ridges, Sierra, and obtaining for one of the most important members of the Comasque chain of Alps its well known Italian name--Il Resegone. Such mountains are not merely successions of irregular peaks, more or less resembling the edge of a much-hacked sword; they are orderly successions of teeth set in one direction, closely resembling those of a somewhat overworn saw, and nearly always produced by successive beds emerging one from beneath the other. [Illustration: FIG. 18.] [Illustration: FIG. 17.] Sec. 22. In all such cases there is an infinitely greater difficulty in accounting for the forms than in explaining the fracture of a single bed. How, and when, and where, were the other portions carried away? Was each bed once continuous over a much larger space from the point where its edge is now broken off, or have such beds slipped back into some gulf behind them? It is very easy for geologists to speak generally of elevation and convulsion, but very difficult to explain what sort of convulsion it could be which passed forward from the edge of one bed to the edge of another, and broke the required portion off each without disturbing the rest. Try the experiment in the simplest way: put half a dozen of hard captain's biscuits in a sloping position on a table, and then try, as they lie, to break the edge of each, one by one, without disturbing the rest. At least, you will have to raise the edge before you can break it; to put your hand underneath, between it and the next biscuit, before you can get any purchase on it. What force was it that put its fingers between one bed of limestone 600 feet thick and the next beneath? If you try to break the biscuits by a blow from above, observe the necessary force of your blow, and then conceive, if you can, the sort of hammer that was required to break the 600 feet of rock through in the same way. But, also, you will, ten to one, break two biscuits at the same time. Now, in these serrated formations, two biscuits are _never_ broken at the same time. There is no appearance of the slightest jar having taken place affecting the bed beneath. If there be, a huge cliff or gorge is formed at that spot, not a sierra. Thus, in Fig. 18, the beds are affected throughout their united body by the shock which formed the ravine at _a_; but they are broken, one by one, into the cliffs at _b_ and _c_. Sometimes one is tempted to think that they must have been slipped back, one from off the other; but there is never any appearance of friction having taken place on their exposed surfaces; in the plurality of instances their continuance or rise from their roots in waves (see Fig. 16 above) renders the thing utterly impossible; and in the few instances which have been known of such action actually taking