Ch. XIV.] 



IN MINERAL CHAEACTEE. 



r >A( 



09 



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 sand banks, 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 



may 



5 



been suspended for one or more geological periods, in the 



made 



ages. 



If deposition be renewed after a long interval, the new 



om 



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 



distant country has been altered. In that 



some 



become 



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, ma} 

 have been formed by subsidence ; and other fluctuations may 

 have occurred, by which the 



T 



materials 



thence by rivers to the sea have acquired a distinct mineral 

 character. 



It is well known that the stream of the Mississippi is 



charged with sediment of a different colour from that of the 



