File:The Quarterly journal of the Geological Society of London (12711464723).jpg

1854..
SHARPE — STRUCTURE OF MONT BLANC.
21
of one great operation, of which we must carefully study the effects,
before we can hope to learn its causes or nature.
Let us now turn to the line of valleys, which, under the names of
the Allee Blanche and the Val Ferret, bounds the eastern side of the
Mont Blanc range. The western side of the Alle'e Blanche is in a
great measure masked by the enormous moraines of the great glaciers
which descend from Mont Blanc, and the various interesting phseno-
mena connected with these somewhat distracted my attention from
the geology of the valley ; but I sketched the section shown in
fig. 2, on the west or Mont Blanc side of the valley, a little above
Fig. 2.
Slaty limestone
Slate.
Calcareous
conglomerate.
Slate.
90" 85° 80" 70"
the Lac de Combal. Slates of various characters rest conformably
on a bed of calcareous conglomerate, in which the cleavage is very
obscure ; this rests on a thick formation of slaty limestone, with
mica lying on the planes of cleavage. The beds all dip conformably
to the S.E. at about the angle of 20°, and are consequently resting
upon the gneiss of Mont Blanc. The cleavage strikes N. 25° E.,
dipping near the mountain towards the E. 25° S. at high angles, but
is vertical in the limestone at the side of the valley ; this is on the
line of the western axis of vertical foliation of the gneiss of Mont
Blanc, and connects that line with the vertical cleavage of the Col de
la Seigne*. These beds doubtless belong to the Jurassic series of the
Col du Bonhomme, and may owe their more metamorphic character
to their proximity to the gneiss ; they cross the valley near the
chapel ; they are probably separated from the gneiss by a metamor-
phic siliceous slate, which is seen on the west side of the valley below
the Glacier de 1' Allee Blanche.
I examined the Piedmontese Val Ferret rather more in detail ; the
position of the rocks on the north side of the Col Ferret is shown in
Sect. 3, PI. I. A thick bed of quartz rock rests upon the gneiss at

• Saussure, § 845, mentions two pyramidal hills of a similar micaceous lime-
  

stone near the head of the Allee Blanche with highly inclined beds ; he doubtless