Sec. 15. In the third paragraph of the last chapter we had occasion to refer to the junction of the slaty and compact crystallines at the roots of the aiguilles. It will be seen in the figure there given, that this change is not sudden, but gradated. The rocks to be joined are of the two types represented in Fig. 3, p. 106 (for convenience' sake I shall in the rest of this chapter call the slaty rock gneiss, and the compact rock protogine, its usual French name). Fig. 55 shows the general manner of junction, beds of gneiss occurring in the middle of the protogine, and of protogine in the gneiss; sometimes one touching the other so closely, that a hammer-stroke breaks off a piece of both; sometimes one passing into the other by a gradual change, like the zones of a rainbow; the only general phenomenon being this, that the higher up the hill the gneiss is, the harder it is (so that while it often yields to the pressure of the finger down in the valley, on the Montanvert it is nearly as hard as protogine); and, on the other hand, the lower down the hill, or the nearer the gneiss, the protogine is, the finer it is in grain. But still the actual transition from one to the other is usually within a few fathoms; and it is that transition, and the preparation for it, which causes the great step, or jag, on the flank of the chain, and forms the tops of the Aiguille Bouchard, Charmoz ridges, Tapia, Montagne de la Cote, Montagne de Taconay, and Aiguille du Goute. [Illustration: FIG. 55.] [Illustration: FIG. 56.] [Illustration: FIG. 57.] [Illustration: FIG. 58.]