y 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 color from that of the Arkansas and Red Rivers,
which are tinged with red mud, derived from rocks of porphyry in "the
far west." The waters of the Uruguay, says Darwin, draining a granitic
country, are clear and black, those of the Parana, red.[258] 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.[259] 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
neighboring coasts, the growth of new deltas, the increase of coral
reefs, and other changes.

_Why successive sedimentary groups contain distinct fossils._--If, in
the next place, we assume, for reasons before stated, a continual
extinction of species and introduction of others into the globe, it will
then follow that the fossils of strata formed at two distant periods on
the same spot, will differ even more certainly than the mineral
composition of the same. For rocks of the same kind have sometimes been
reproduced in the same district after a long interval of time, whereas
there are no facts leading to the opinion that species which have once
died out have ever been reproduced. The submergence then of land must be
often attended by the commencement of a new class of sedimentary
deposits, characterized by a new set of fossil animals and plants, while
the reconversion of the bed of the sea into land may arrest at once and
for an indefinite time the formation of geological monuments. Should the
land again sink, strata will again be formed; but one or many entire
revolutions in animal or vegetable life may have been completed in the
interval.

_Conditions requisite for the original completeness of a fossiliferous
series._--If we infer, for reasons before explained, that fluctuations
in the animate world are brought about