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new junctions occurring from time to time; so that Eridanus, "the king
of rivers," will continually boast a greater number of tributaries. The
Ganges and the Brahmapootra have perhaps become partially confluent in
the same delta within the historical, or at least within the human era;
and the date of the junction of the Red River and the Mississippi would,
in all likelihood, have been known, if America had not been so recently
discovered. The union of the Tigris and the Euphrates must undoubtedly
have been one of the modern geographical changes of our Earth, for Col.
Rawlinson informs me that the delta of those rivers has advanced two
miles in the last sixty years, and is supposed to have encroached about
forty miles upon the Gulf of Persia in the course of the last
twenty-five centuries.

When the deltas of rivers, having many mouths, converge, a partial union
at first takes place by the confluence of some one or more of their
arms; but it is not until the main trunks are connected above the head
of the common delta, that a complete intermixture of their joint waters
and sediment takes place. The union, therefore, of the Po and Adige, and
of the Ganges and Brahmapootra, is still incomplete. If we reflect on
the geographical extent of surface drained by rivers such as now enter
the Bay of Bengal, and then consider how complete the blending together
of the greater part of their transported matter has already become, and
throughout how vast a delta it is spread by numerous arms, we no longer
feel so much surprise at the area occupied by some ancient formations of
homogeneous mineral composition. But our surprise will be still farther
lessened, when we afterwards inquire (ch. 21) into the action of tides
and currents in disseminating sediment.

_Age of existing deltas._--If we could take for granted, that the
relative level of land and sea had remained stationary ever since all
the existing deltas began to be formed--could we assume that their
growth commenced at one and the same instant when the present continents
acquired their actual shape--we might understand the language of
geologists who speak of "the epoch of existing continents." They
endeavor to calculate the age of deltas from this imaginary fixed
period; and they calculate the gain of new land upon the sea, at the
mouths of rivers, as having begun everywhere simultaneously. But the
more we study the history of deltas, the more we become convinced that
upwar