Masterpieces of Science 



shifting of these areas from one region to 

 another. 



First, then, variations in the site of sedimen- 

 tary deposition are brought about independently 

 of subterranean movements. There is always a 

 slight change from year to year, or from century 

 to century. The sediment of the Rhone, for ex- 

 ample, thrown into the Lake of Geneva, is now 

 conveyed to a spot a mile and a half distant from 

 that where it accumulated in the tenth century, 

 and six miles from the point where the delta 

 began originally to form. We may look forward 

 to the period when this lake will be filled up, and 

 then the distribution of the transported matter 

 will be suddenly altered, for the mud and sand 

 brought down from the Alps will thenceforth, 

 instead of being deposited near Geneva, be carried 

 nearly two hundred miles southwards, where the 

 Rhone enters the Mediterranean. 



In the deltas of large rivers, such as those of 

 the Ganges and Indus, the mud is first carried 

 down for many centuries through one arm, and 

 on this being stopped up it is discharged by 

 another, and may then enter the sea at a point 

 fifty or one hundred miles distant from its first 

 receptacle. The direction of marine currents 

 is also liable to be changed by various accidents, 

 as by the heaping up of new sandbanks, or the 

 wearing away of cliffs and promontories. 



But, secondly, all these causes of fluctuation 

 in the sedimentary areas are entirely subordinate 

 to those great upward or downward movements 

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