12 REMARKABLE POSITION
having once formed a lake, before the
waters of the Arve escaped, as at present,
through a lateral chasm to Pont Pelissier.
The occurrence of valleys closed in at both
ends is not unfrequent in Savoy. 1 have
before remarked that such valleys could
not have been formed by submarine currents,
or by rivers running through th em ;
this is evident from their present structure.
The base of the mountain of the Col de
Balme is granitic, but the north and northwest
side of the mountain, are composed of
nearly vertical beds of sandstone, pudding-
stone, micaceous sandstone, nearly resembling
mica-slate, dark soft schist, schisty
grey limestone, and slate. The sandstone,
puddingstone, and micaceous sandstone alternate
; they range nearly north-east by
north. Standing on the summit o f the
Col de Balme, one of these beds, of vast
thickness, is seen plunging down into the
valley to the north-west, and is nearly vertical
and uncovered by any other bed, to
the depth of fifteen hundred or two thousand
feet. Were any decomposition, or
erosion of the lower part to take place, the
whole bed must fall into the valley. I could
OF THE STRATA. 13
not approach sufficiently near to ascertain
its nature, but I believe it to be the schisty
limestone described by Saussure. I did
not see the beds of puddingstone which
are associated with the vertical beds of
this mountain, as in situ it would have required
another day to examine the north
side of the Col de Balme, towards the Va-
lorsine. The beds of which I obtained
specimens, are greyish or reddish schist,
with rather a fine paste, and containing
particles of mica, and would, in the Wernerian
nomenclature, be denominated a
grey-wacke schist. In the paste are imbedded
numerous holders and fragments
of gneiss, mica-slate, and quartz, varying
in size from that of a pea, or smaller, to
seven or eight inches in diameter; some
are perfectly rounded, others are angular,
and some have a rhomboidal form, derived
from the cleavage of the rocks, from
whence they were originally broken. Many
of these beds are absolutely vertical, others
have a slight inclination; they alternate
with sandstone, and thin beds of schist, in
which are few or no fragments. The total
thickness of the beds of schist and puddingstone
is about 620 feet, and they may