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In the fifteenth chapter we spoke of the nature of combats in mountains, and in the sixteenth of the use to be made of them by strategy, and in so doing we often came upon the idea of mountain defence, without stopping to consider the form and details of such a measure. We shall now examine it more closely. As mountain systems frequently extend like streaks or belts over the surface of the earth, and form the division between streams flowing in different directions, consequently the separation between whole water systems, and as this general form repeats itself in the parts composing that whole, inasmuch as these parts diverge from the main chain in branches or ridges, and then form the separation between lesser water systems; hence the idea of a system of mountain defence has naturally founded itself in the first instance, and afterwards developed itself, upon the conception of the general form of mountains, that of an obstacle, like a great barrier, having greater length than breadth. Although geologists are not yet agreed as to the origin of mountains and the laws of their formation, still in every case the course of the waters indicates in the shortest and surest manner the general form of the system, whether the action of the water has contributed to give that general form (according to the aqueous theory), or that the course of the water is a consequence of the form of the system itself. It was, therefore, very natural again, in devising a system of mountain defence, to take the course of the waters as a guide, as those courses form a natural series of levels, from which we can obtain both the general height and the general profile of the mountain, while the valleys formed by the streams present also the best means of access to the heights, because so much of the effect of the erosive and alluvial action of the water is permanent, that the inequalities of the slopes of the mountain are smoothed down by it to one regular slope. Hence, therefore, the idea of mountain defence would assume that, when a mountain ran about parallel with the front to be defended, it was to be regarded as a great obstacle to approach, as a kind of rampart, the gates of which were formed by the valleys. The real defence was then to be made on the crest of this rampart, (that is, on the edge of the plateau which crowned the mountain) and cut the valleys transversely. If the line of the principal mountain-chain formed somewhat of a right angle with the front of defence, then one of the principal branches would be selected to be used instead; thus the line chosen would be parallel to one of the principal valleys, and run up to the principal ridge, which might be regarded as the extremity.