UNIFORMITY OF CHANGE 423 



delta began originally to form. We may look forward to the 

 period when this lake will be filled up, and then the distribu- 

 tion of the transported matter will be suddenly altered, for 

 the mud and sand brought down from the Alps will thence- 

 forth, instead of being deposited near Geneva, be carried 

 nearly 200 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 dis- 

 charged by another, and may then enter the sea at a point 

 50 or 100 miles distant from its first receptacle. The direc- 

 tion 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 sedi- 

 mentary areas are entirely subordinate to those great upward 

 or downward movements of land, which will presently be 

 spoken of, as prevailing over large tracts of the globe. By 

 such elevation or subsidence certain spaces are gradually 

 submerged, or made gradually to emerge: in the one case 

 sedimentary deposition may be suddenly renewed after hav- 

 ing been suspended for one or more geological periods, in the 

 other as suddenly made to cease after having continued for 

 ages. 



If deposition be renewed after a long interval, the new 

 strata will usually differ greatly from the sedimentary rocks 

 previously formed in the same place, and especially if the 

 older rocks have suffered derangement, which implies a 

 change in the physical geography of the district since the 

 previous conveyance of sediment to the same spot. It may 

 happen, however, that, even where the two groups, the 

 superior and the inferior, are horizontal and conformable to 

 each other, they may still differ entirely in mineral charac- 

 ter, because, since the origin of the older formation, the 

 geography of some distant country has been altered. In that 

 country rocks before concealed may have become exposed by 

 denudation; volcanos may have burst out and covered the 

 surface with scoriae and lava ; or new lakes, intercepting the 

 sediment previously conveyed from the upper country, may 



