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24 GEOLOGY OF
have been raised together. The upper part
of Mont Blanc is a different kind of granite,
in which soft greenish talc supplies the place
of mica. To this combination the name
of protogine has been very improperly
given ; it woidd be quite sufficient to describe
it as a talcous granite, to distinguish
it from common granite that contains mica.
Saussure, after a very extended examination
o f the granite of the Aiguilles of Mont
Blanc, and the southern side of the valley
of Chamouny, ascertained that the general
range of the beds was nearly north-east,
and south-west, or parallel to the direction
of the valley, and they become more elevated
towards the middle of the mountains,
where they are quite vertical. All the
mountains on the south side of Chamouny
may be regarded as united, and forming
one mass, from which detached pinnacles
and summits rise, as from an elevated plain.
The average height of this mass is 5000
feet, and from the central part, which is
granite, numerous pyramids and spires of
granite shoot up to twice that height above
the valley. ,
The northern side of the valley is composed
of mountains principally of granite
and mica-slate; the beds of which are not
exactly parallel with the valley, but range in
a more northerlv direction : the central beds
are vertical.
The mountain which closes the southwestern
end of the valley, is composed of a
blueish-grey limestone and slate ; th e beds
ranging south-west and north-east, or in the
direction of the valley, and are muchinclined.
The Col de Balme, or mountain which closes
the north-east end of the valley, has been
already described: its beds are nearly vertical,
their direction is rather more northerly,
than the general direction of the valley.
This valley may therefo/e be regarded as a
deep trough about twelve miles in length
at the bottom, having no original outlet at
either end ; but the waters have worked a
lateral passage in a rock of soft slate, and
near its junction with a very hard granite,
forming a deep chasm which extends four
miles or more, from near Ouches to Pont
Pelissier. Before this chasm was opened,
the valley of Chamouny must have been a
lake, surrounded every where by steep
mountains. As the height of the bottom o f
the valley above the level of the sea exceeds
considerably 3000 English f e e t; it is
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