File:The Quarterly journal of the Geological Society of London (13204819044).jpg

EEV. A. TRYING ON THE
CQ
ZQ
(2) Again, it may well have hap-
pened that in some of these later
movements of the mountains, lateral
thrusts may have acted vertically,
or nearly so, to older lines of valley.
The effect of such forces would be,
if the resultant force was not too
deeply seated, to partly close in the
valleys upon which they were
brought to bear, until the sides of
the valleys in their narrowest parts
came together, leaving the wider
portions to form true rock-basins,
which, getting filled with water
from the surface-drainage, would, of
course, form lakes. The direction
of the principal lines of flexure of
the Jura chain affords a strong pre-
sumption in favour of the hypo-
thesis that in this way the valley of
the Rhone below Geneva may have
been partly closed in. This alone
would account for so much of that
lake as lies in a true rock-basin.
Part of its depth, however, it owes
to the delta of the Arve, which has
been brought down from the Mont-
Blanc region*.
(3) There is a phenomenon f well
known to mining-engineers as a
"creep." In old coal-workings it
is found that the vertical pressure
of the superincumbent strata, acting
upon the strata immediately below
the galleries where the coal-seams
have been removed, causes the floor
of the mine to rise, so as in time to
completely fill up the disused work-
ings. Such a resolution of vertical
pressure is possible where more mas-
sive strata, as often occurs in the
lake-regions of the Alps, rest upon
strata of a more yielding nature on
opposite sides of a gorge-like valley.
In such cases the erosion of the
valley being continued down to the
lower strata, these, being at the

• Vide ' Les Causes actuelles en Gre'olo-
  

gie,' by Meunier, pp. 203, 204.