and the mouth of the Mississippi, is ahout 500 feet. Borings have
been made near Hew Orleans to a depth of 600 feet, without reaching
the bottom of the alluvial matter; so that the depth of the delta of
the Mississippi may he safely taken at 500 feet. The entire alluvial
- plain is 30,000 square miles in extent, and the smallest complement
of time required for its formation has been estimated at 100,000 years.*
This calculation merely embraces the deposits made by the river since
it ran in its present channel; but such an antiquity dwindles into
utter insignificance when we consider the geological features of the
country. The bluffs which hound the valley of the Mississippi rise
in many places to a height of 250 feet, and consist of loam containing
shells of various species still inhabiting the country. These shells
are accompanied with the remains of the mastodon, elephant, and
tapir, the megalonyx, and other megatheroid animals, together with
the horse, ox, and other mammalia, mostly of extinct species. These
fluffs must have belonged to an ancient plain of ages long anterior
to that through which the Mississippi now flows, and which was inhabited
by occupants of land and fresh-water shells agreeing with those
now existing, and by quadrupeds now mostly extinct, f
The plain on which the city of Hew Orleans is built, rises only nine
feet above the sea; and excavations are often made far below the
level of the Gulf of Mexico. In these sections, several successive
growths of cypress timber have been brought to light. In digging
the foundations for the gas-works, the Irish spadesmen, finding they
had to cut through timber instead of soil, gave up the work, and were
replaced by a corps of 'Kentucky axe-men, who hewed their way
downwards through four successive growths of timber —the lowest
so old that it cut like cheese. Abrasions of the river-banks show
similar growths of sunken timber; while stately live-oaks, flourishing
on the hank directly above them, are living witnesses that the soil
has not changed its level for ages. Messrs. Dickeson and Brown
have traced no less than ten distinct cypress forests at different levels
below the present surface, in parts of Louisiana where the range between
high and low water is much greater than it is at Sew Orleans.
These groups of trees (the live-oaks on the banks, and the successive
cypress beds beneath,) are arranged vertically above each other, and
are seen to great advantage in many places in the vicinity of Hew
Orleans.
Dr. Bennet DowlerJ has made an ingenious calculation of the last
emergence of the site of that city, in which these cypress forests play
* LyeU’s Principles of Geology, Cap. xv. f Lyell’s Second Visit, Cap. xxxiv.
J Bennet Dowler: Tableaux of New Orleans, 1852.
an important part. He divides the history of this event into three
era8: __l. The era of colossal grasses, trembling prairies, &c., as seen
in the lagoons, lakes, and sea-coast. 2. The era of th^cypress basins.
3. The era of the present live-oak platform. Existing: types, from
the Balize to the highlands, show that these belts were successively
developed from the water in the order we have named: the grass
preceding the cypress, and the cypress being succeeded by the live-
oak. Supposing an elevation of five inches in a century, (which is
about the rate recorded for the accumulation of detrital deposits in
the valley of the Kile, during seventeen centuries, by the nilometer
mentioned by Strabo,) we shall have 1500 years'for the era of aquatic
plants until the appearance of the first cypress forest; or, in other
words, for the elevation of the grass zone to the condition of a cypress
basin. .
Cypress trees of ten feet ill diameter are not uncommon m the
swamps of Louisiana; and one of that size was found in the lowest
bed of the excavation at the gas-works in Hew Orleans. Taking ten
feet to represent the size of one generation of trees, we shall have a
period of 5700 years as the age of the oldest trees now growing in
the basin. Messrs-. Dickeson and Brown, in examining the cypress
timber of Louisiana and Mississippi, found that they measured from
95, to 120 rings of annual growth to an inch: and, according to the
lower ratio, a tree of ten feet in diameter will yield 5700 rings of
annual growth. Though many generations of such trees may have
grown and perished in the present cypress region, Dr. Dowler, to
avoid all ground of cavil, has assumed only two consecutive growths,
including the One now standing: this gives us, as the age of two
generations of cypress trees, 11,400 years.
The maximum age of the oldest tree growing on the live-oak platform
is estimated at 1500 years, and only one generation is counted.
These data yield the following table: —
“ Geological Chronology of the last emergence of the present site of New Orleans.
Tears.
Era of ngoa/deV®“*8 ........................................................................™........... 1.600
Era of cypress basin................ ........ « v — 11. 0
Era of live-oak platform................................................................................................... 1,5
Total period of elevation............................................ 14,400”
Each of these sunken forests must have had a period of rest and
gradual depression, estimated as equal to 1500 years for tlm duration
of the live-oak era, which, of course, occurred but once in the
series. We shall then certainly be within bounds, if we assume the
period of such elevation to have been equivalent to the one above
43