recession is a
matter of interest, for it takes place exactly on the line above spoken
of, where the slaty crystallines of the crest join the compact
crystallines of the aiguilles; at which junction a correspondent chasm
or recession, of some kind or another, takes place along the whole front
of Mont Blanc.

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.]

Sec. 16. But what most puzzled me was the intense _straightness_ of the
lines of the gneiss beds, dipping, as it seemed, under the Mont Blanc.
For it has been a chief theory with geologists that these central
protogine rocks have once been in fusion, and have risen up in molten
fury, overturning and altering all the rocks around. But every day, as
I looked at the crested flanks of the Mont Blanc, I saw more plainly the
exquisite _regularity_ of the slopes of the beds, ruled, it seemed, with
an architect's rule, along the edge of their every flake from the
s