2IO CLASSICS OF MODERN SCIENCE 

 movements of lands, which will be presently spoken of, as prevailing 

 over large tracts of the globe. By such elevation or subsidence cer- 

 tain spaces are gradually submerged, or made gradually to emerge : in 

 the one case sedimentary deposition may be suddenly renewed after 

 having 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 character, 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 be- 

 come exposed by denudation ; volcanoes 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 have 

 been formed by subsidence ; and other fluctuations may have occurred, 

 by which the materials brought down from 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 Arkansas and Red 

 Rivers, which are tinged with red mud, derived from rocks of por- 

 phyry and red gypseous clays in "the far west." The waters of the 

 Uruguay, says Darwin, draining a granitic country, are clear and 

 black, those of the Parana, red. The mud with which the Indus is 

 loaded, says Burnes, is of a clayey hue, that of the Chenab, on the 

 other hand, is reddish, that of the Sutlej is more pale. The same 

 causes which make these several rivers, sometimes situated at no great 

 distance the one from the other, to differ greatly in the character of 

 their sediment, will make the waters draining the same country at 

 different epochs, especially before and after great revolutions in physi- 

 cal geography, to be entirely dissimilar. It is scarcely necessary to 

 add that marine currents will be affected in an analogous manner in 

 consequence of the formation of new shoals, the emergence of new 

 islands, the subsidence of others, the gradual waste of neighbouring 



