Sec. 2. I begin with the Aiguilles. In Fig. 24, p. 170, at _a_, it was assumed that the mass was raised highest merely where the elevating force was greatest, being of one substance with the bank or cliff below. But it hardly ever _is_ of the same substance. Almost always it is of compact crystallines, and the bank of slaty crystallines; or if it be of slaty crystallines the bank is of slaty coherents. The bank is almost always the softer of the two.[57] Is not this very marvellous? Is it not exactly as if the substance had been prepared soft or hard with a sculpturesque view to what had to be done with it; soft, for the glacier to mould, and the torrent to divide; hard, to stand for ever, central in mountain majesty. [Illustration: FIG. 25.] Sec. 3. Next, then, comes the question, How do these compact crystallines and slaty crystallines join each other? It has long been a well recognized fact in the science of geology, that the most important mountain ranges lift up and sustain upon their sides the beds of rock which form the inferior groups of hills around them in the manner roughly shown in the section Fig. 25, where the dark mass stands for the hard rock of the great mountains (crystallines), and the lighter lines at the side of it indicate the prevalent direction of the beds in the neighboring hills (coherents), while the spotted portions represent the gravel and sand of which the great plains are usually composed. But it has not been so universally recognized, though long ago pointed out by De Saussure, that the great central groups are often themselves composed of beds lying in a precisely opposite direction; so that if we analyze carefully the structure of the dark mass in the centre of Fig. 25, we shall find it arranged in lines which slope downwards to the centre; the flanks of it being of slaty crystalline rock, and the summit of compact crystallines, as at _a_, Fig. 26. [Illustration: FIG. 26.] In speaking of the sculpture of the central peaks in the last chapter, I made no reference to the _nature_ of the rocks in the banks on which they stood. The diagram at _a_, Fig. 27, as representative of the original condition, and _b_, of the resultant condition will, compared with Fig. 24, p. 170, more completely illustrate the change.[58] [Illustration: FIG. 27.] [Illustration: FIG. 28.]