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SIR CHARLES LYELL 



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 porphyry and red gypseous clays in 

 'the far west/ The waters of the Uruguay, says Darwm, 

 draining a granitic country, are clear and black, those of the 

 Parana, red. 1 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 physical 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 coasts, the growth of new deltas, the increase of 

 coral reefs, volcanic eruptions, and other changes. 



Uniformity of change considered, secondly, in reference 

 to the living creation. Secondly, in regard to the vicissi- 

 tudes of the living creation, all are agreed that the successive 

 groups of sedimentary strata found in the earth's crust are 

 not only dissimilar in mineral composition for reasons above 

 alluded to, but are likewise distinguishable from each other 

 by their organic remains. The general inference drawn 

 from the study and comparison of the various groups, 

 arranged in chronological order, is this: that at successive 

 periods distinct tribes of animals and plants have inhabited 

 the land and waters, and that the organic types of the newer 

 formations are more analogous to species now existing than 

 those of more ancient rocks. If we then turn to the present 

 state of the animate creation, and enquire whether it has 



1 Darwin's Journal, p. 163, and edit., p. 139. 

 Journ. Roy. Geograph. Soc., vol. iii., p. 142. 



