00 square
miles. The space south of this in the bay, where sediment is thrown
down, may be 300 miles from E. to W. by 150 N. and S., or 45,000 square
miles, which, added to the former, gives a surface of 65,000 square
miles, over which the sediment is spread out by the two rivers. Suppose
then the solid matter to amount to 40,000 millions of cubic feet per
annum, the deposit, he observes, must be continued for forty-five years
and three-tenths to raise the whole area a height of one foot, or 13,600
years to raise it 300 feet; and this, as we have seen, is much less than
the thickness of the fluviatile strata actually penetrated, (and the
bottom not reached) by the auger at Calcutta.

Nevertheless we can by no means deduce from these data alone, what will
be the future rate of advance of the delta, nor even predict whether the
land will gain on the sea, or remain stationary. At the end of 13,000
years the bay may be less shallow than now, provided a moderate
depression, corresponding to that experienced in part of Greenland for
many centuries shall take place (see chap. 30). A subsidence quite
insensible to the inhabitants of Bengal, not exceeding two feet three
inches in a century, would be more than sufficient to counterbalance all
the efforts of the two mighty rivers to extend the limits of their
delta. We have seen that the Artesian borings at Calcutta attest, what
the vast depth of the "swatch" may also in all likelihood indicate, that
the antagonist force of subsidence has predominated for ages over the
influx of fluviatile mud, preventing it from raising the plains of
Bengal, or from filling up a larger portion of the bay.

CONCLUDING REMARKS ON DELTAS.

_Convergence of deltas._--If we possessed an accurate series of maps of
the Adriatic for many thousand years, our retrospect would, without
doubt, carry us gradually back to the time when the number of rivers
descending from the mountains into that gulf by independent deltas was
far greater in number. The deltas of the Po and the Adige, for instance,
would separate themselves within the _recent_ era, as, in all
probability, would those of the Isonzo and the Torre. If, on the other
hand, we speculate on future changes, we may anticipate the period when
the number of deltas will greatly diminish; for the Po cannot continue
to encroach at the rate of a mile in a hundred years, and other rivers
to gain as much in six or seven centuries upon the shallow gulf, witho