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 



