THE MOUNTAINS OF SWITZERLAND. 
91 
to erosion, that the Maderaner 
valley has been cut out of the Crys- 
talline rocks, f, and was once 
covered by the Jurassic strata j, 
which must have formerly passed in 
a great arch over what is now the 
valley. 
Again it is clear (Fig. 25) that 
a great thickness of Crystalline rock 
has been removed from the summit 
of Mont Blanc. No doubt (see ante, 
p. 28) many thousand feet had been 
removed before the deposition of 
the Secondary strata. But even since 
its elevation the amount of erosion 
of the Granite itself has been con- 
siderable. How much we do not 
know, but 500 metres would pro- 
bably be a moderate estimate. To 
this must be added the Crystalline 
Schists, say 1000 metres, and the 
Sedimentary rocks, which from what 
we know of their thickness elsewhere 
cannot be taken at less than 3000 
metres. This therefore gives 4500 
metres, or say 14,000 feet, which 
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