36 On the Delta and Alluvial Deposits of the Mississippi. 
north and south, and thirty east and west, and is for the most 
part submerged. Many dead trees are still standing erect in the 
swamps, a far greater number lie prostrate. Even on the dry 
ground in the vicinity, all the forest trees which are of prior date- 
to 1811, are leafless: they are supposed to have been killed by 
the loosening of their roots by the repeated shocks of 181 1, 12. 
Numerous rents are also observable in the ground where it opened 
in 1811; and many “sink holes,” or cavities, from ten to thirty 
yards wide and twenty feet or more in depth, now interrupt the 
general level of the plain, which were formed by the spouting 
out of large quantities of sand and mud during the earthquake. 
In attempting to compute the minimum of time required for the 
accumulation of the alluvial matter in the delta and valley of the 
Mississippi, Mr. Lyell referred to a series of experiments, made by 
Dr. Riddell, at New Orleans, showing that the mean annual pro- 
portion of sediment in the river was, to the water r273 Nn Weight, 
or about ;;'5; in volume. From the observations of the same 
of water, are deduced. In assuming 528 feet (or the tenth of a 
mile) as the probable thickness of the deposit of mud and sand 
-— ? 
timated 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 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 
joss of matter, owing to the finer particles of mud not settling 
at the mouth of the river, but being Swept out far to sea, and 
years, must be insignificant, in a geological point of view, since the 
date, ) and which are from 50 to 250 feet in perpendicular height, 
Consist in great part of loam, containing land, fluviatile, and la- 
custrine shells of species still inhabiting the same country. ‘These 
fossil shells, oceurring in a deposit resembling the loess of the 
