mass of alluvial matter, is a
question which may well excite the curiosity of every geologist.
Sufficient data have not yet been obtained to afford a full and
satisfactory answer to the inquiry, but some approximation may already
be made to the minimum of time required.

When I visited New Orleans, in February, 1846, I found that Dr. Riddell
had made numerous experiments to ascertain the proportion of sediment
contained in the waters of the Mississippi; and he concluded that the
mean annual amount of solid matter was to the water as 1/1245 in weight,
or about 1/3000 in volume.[367] From the observations of the same
gentleman, and those of Dr. Carpenter and Mr. Forshey, an eminent
engineer, to whom I have before alluded, the average width, depth, and
velocity of the Mississippi, and thence the mean annual discharge of
water were deduced. I assumed 528 feet, or the tenth of a mile, as the
probable thickness of the deposit of mud and sand in the delta; founding
my conjecture chiefly on the depth of the Gulf of Mexico, between the
southern point of Florida and the Balize, which equals on an average 100
fathoms, and partly on some borings 600 feet deep in the delta, near
Lake Pontchartrain, north of New Orleans, in which the bottom of the
alluvial matter is said not to have been reached. The area of the delta
being about 13,600 square statute miles, and the quantity of solid
matter annually brought down by the river 3,702,758,400 cubic feet, it
must have taken 67,000 years for the formation of the whole; and if the
alluvial matter of the plain above be 264 feet deep, or half that of the
delta,[368] it must have required 33,500 more years for its
accumulation, even if its area be estimated as only equal to that of the
delta, whereas it is in fact larger. If some deduction be made from the
time here stated, in consequence of the effect of the drift-wood, which
must have aided in filling up more rapidly the space above alluded to, a
far more important allowance must be made on the other hand, for the
loss of matter, owing to the finer particles of mud not settling at the
mouths of the river, but being swept out far to sea during the
predominant action of the tides, and the waves in the winter months,
when the current of fresh water is feeble. Yet however vast the time
during which the Mississippi has been transporting its earthy burden to
the ocean, the whole period, though far exceeding, perhaps, 100,000
years, must be insign