E. W. Hilgard—Geology of Lower Louisiana. 87 
bers of the Port Hudson series, whose floor stratum, with its 
superimposed generations of cypress stumps, indicate a slow 
secular subsidence. The velocity of the latter seems gradually 
to have increased until the growth of old trees became impos- 
sible, and finally, in stratum No. 3 of the Port Hudson profile, 
we again meet the evidences of currents moving sand, pebbles, 
and drift-wood. 
Then follows the Loess proper, a deposit utterly devoid, in 
Mississippi and Louisiana, of any evidences of fluviatile action 
—a uniform silt even in profiles of 80 feet, with scarcely a ves- 
tige of stratification, and none but terrestrial fossils. 
The precise circumstances under which such a deposit could 
be formed, are perhaps a little obscure. There must have been 
such a depression of the whole country as to transform the 
immediate valley of the Mississippi, as far as Keokuk, as well 
as the valleys of the larger tributaries, into estuaries of the 
Gulf of Mexico, containing a mass of water too great to be 
sensibly affected by the variations now causing the annual 
overflows of those rivers (for otherwise the deposits must have 
own lines of deposition), yet possessing a gentle flow above 
(since the materials of the bluff formation of Missouri and 
Indiana exhibit signs of fluviatile action) ; quite fresh in its 
upper portions (where fluviatile shells are found), but rendered 
unfit for the life of either a fresh or salt-water fauna by an 
admixture of sea-water, in its lower and almost stagnant por- 
tion, at tide level ; and deriving its vestiges of animal life only 
from the “ offscourings” of the adjoining unsubmerged lands. 
Sir Chas. Lyell inclines to consider the Loess as the 
absence of stratification from such deposits having, appar- 
ently, an analogue in the alluvial deposits of the Nile. But 
The Nile mud is each year derived from 
ssissippi, on the contrary, the deposits of different 
inundations are readily istinguished by the inhabi- 
souri, Ohio, Arkansas or Red river happen to have furnished 
the main influx. The absence of any such differences from the 
_ Loess can only be explained on the assumption that the mass of 
