28 
SHIFTING OF THE AREAS 
[Ch. III. 
near Geneva, be carried nearly two hundred miles southwards, 
where the Rhone enters the Mediterranean. 
The additional matter thus borne down to the lower delta 
of the Rhone would not only accelerate its increase, but might 
affect the mineral character of the strata there deposited, and 
thus give rise to an upper group, or subdivision of beds, having 
a distinct character. But the filling up of a lake, and the 
consequent transfer of the sediment to a new place, may some- 
times give rise to a more abrupt transition from one group 
to another; as, for example, in a gulf like that of the St. Law- 
rence, where no deposits are now accumulated, the river being 
purged of all its impurities in its previous course through the 
Canadian lakes. Should the lowermost of these lakes be at 
any time filled up with sediment, or laid dry by earthquakes, 
the waters of the river would thenceforth become turbid, and 
strata would begin to be deposited in the gulf, where a new 
formation would immediately overlie the ancient rocks now 
constituting the bottom. In this case there would be an 
abrupt passage from the inferior and more ancient, to the 
newer superimposed formation. 
The same sudden coming on of new sedimentary deposits, 
or the suspension of those which were in progress, must fre- 
quently occur in different submarine basins where there are 
currents which are always liable, in the course of ages, to 
change their direction. Suppose, for instance, a sea to be 
filling up in the same manner as the Adriatic, by the influx of 
the Po, Adige, and other rivers. The deltas, after advancing 
and converging, may at last come within the action of a 
transverse current, which may arrest the further deposition of 
matter, and sweep it away to a distant point. Such a current 
now appears to prey upon the delta of the Nile, and to carry 
eastward the annual accessions of sediment that once added 
rapidly to the plains of Egypt. 
On the other hand, if a current charged with sediment vary 
its course, a circumstance which, as we have shown, must hap- 
pen to all of them in the lapse of ages, the accumulation of 
