having formerly, as may be seen by ancient maps, moved
eastward for a long period.

The area of the delta of the combined rivers, for it is impossible now
to distinguish what belongs to each, is considerably more than double
that of the Nile, even if we exclude from the delta a large extent of
low, flat, alluvial plain, doubtless of fluviatile origin, which
stretches more than 100 miles to the hills west of Calcutta (see map,
fig. 25), and much farther in a northerly direction beyond the head of
the great delta. The head of a delta is that point where the first arm
is given off. Above that point a river receives the waters of
tributaries flowing from higher levels; below it, on the contrary, it
gives out portions of its waters to lower levels, through channels which
flow into adjoining swamps, or which run directly to the sea. The
Mississippi, as before described, has a single head, which originated at
an unknown period when the Red River joined it. In the great delta of
Bengal there may be said to be two heads nearly equidistant from the
sea, that of the Ganges (G, map, fig. 25), about 30 miles below
Rajmahal, or 216 statute miles in a direct line from the sea, and that
of the Brahmapootra (B), below Chirapoonjee, where the river issues from
the Khasia mountains, a distance of 224 miles from the Bay of Bengal.

It will appear, by reference to the map, that the great body of fresh
water derived from the two rivers enters the bay on its eastern side;
and that a large part of the delta bordering on the sea is composed of a
labyrinth of rivers and creeks, all filled with salt water, except those
immediately communicating with the Hoogly, or principal arm of the
Ganges. This tract alone, known by the name of the Woods, or Sunderbunds
(more properly Soonderbuns), a wilderness infested by tigers and
crocodiles, is, according to Rennell, equal in extent to the whole
principality of Wales.[369]

On the sea-coast there are eight great openings, each of which has
evidently, at some ancient period, served in its turn as the principal
channel of discharge. Although the flux and reflux of the tide extend
even to the heads of the delta when the rivers are low, yet, when they
are periodically swollen by tropical rains, their volume and velocity
counteract the tidal current, so that, except very near the sea, the ebb
and flow become insensible. During the flood season, therefore, the
Ganges and Brahmapootra almost assume in their del