down with great violence when the tide ebbs. The bay for forty miles
south of Chittagong is so fresh that neither algae nor mangroves will
grow in it. We may, therefore, conceive how effective may be the current
formed by so great a volume of water in dispersing fine mud over a wide
area. Its power is sometimes augmented by the agitation of the bay
during hurricanes in the month of May. The new superficial strata
consists entirely of fine sand and mud; such, at least, are the only
materials which are exposed to view in regular beds on the banks of the
numerous creeks. Neither here or higher up the Ganges, could Dr. Hooker
discover any land or freshwater shells in sections of the banks, which
in the plains higher up sometimes form cliffs eighty feet in height at
low water. In like manner I have stated[373] that I was unable to find
any buried shells in the delta or modern river cliffs of the
Mississippi.

No substance so coarse as gravel occurs in any part of the delta of the
Ganges and Brahmapootra, nor nearer the sea than 400 miles. Yet it is
remarkable that the boring of an Artesian well at Fort William, near
Calcutta, in the years 1835-1840, displayed, at the depth of 120 feet,
clay and sand with pebbles. This boring was carried to a depth of 481
feet below the level of Calcutta, and the geological section obtained in
the operation has been recorded with great care. Under the surface soil,
at a depth of about ten feet, they came to a stiff blue clay about forty
feet in thickness; below which was sandy clay, containing in its lower
portion abundance of decayed vegetable matter, which at the bottom
assumed the character of a stratum of black peat two feet thick. This
peaty mass was considered as a clear indication (like the "dirt-bed" of
Portland) of an ancient terrestrial surface, with a forest or Sunderbund
vegetation. Logs and branches of a red-colored wood occur both above and
immediately below the peat, so little altered that Dr. Wallich was able
to identify them with the Soondri tree, _Heritiera littoralis_, one of
the most prevalent forms, at the base of the delta. Dr. Falconer tells
me that similar peat has been met with at other points round Calcutta at
the depth of nine feet and twenty-five feet. It appears, therefore, that
there has been a sinking down of what was originally land in this
region, to the amount of seventy feet or more perpendicular; for
Calcutta is only a few feet above the level of the sea, an