A. Tylor — Formation of Deltas. 491 



191, and compared with the rainfall at 89|, is about as 1 to 41. This 

 difference arises from the re-evaporation of the rainfall several times 

 before it reaches the Gulf. 



In 1851 the mean proportion of sediment to water for the whole 

 year was -t-gV-FJ *^® maximum was -g-i-p. 



Professor Eiddell's last experiment was --Vt ^J 'height. 



By Mr. Meade's experiments, page 149, it appears that, at Carrol- 

 ton, sand is always moving along the bottom of the Mississippi. An 

 engineer who has resided in a plantation near Fort St. Philip, below 

 New Orleans, describes the newly-deposited material to be sand, and 

 not mud, and that it is observed to move along the bottom of the 

 river at a point within thirty miles of the Gulf of Mexico. At 

 Carrolton the mean temperature of the air in 1851 was 67°-6, the 

 river water being 63°-9. The mean temperature of the Mississippi 

 increases 3^ between Memphis and Carrolton. 



Taking the present rainfall at 41-5 in., and that in the Pluvial 

 Period at 300 in. (p. 9, Quart. Journ., vol. xxv.), the ratio is 7-23 to 1, 

 or sevenfold. If the solid matter in suspension was seven times 

 coarser than at present, on the average, we have a new measure of 

 the power of denudation and of the extent of rainfall in the Pluvial 

 Period, for the velocity and quantity of water flowing at the same 

 slope must be in a definite proportion to the coarseness of the 

 materials carried by the water, and vice versa. If 300 inches of 

 rain fell in large storms, there might have been floods carrying 125 

 times the present quantity of water, an amount deduced by the 

 autlior from observing the form and dimensions of the Quaternary 

 deposits in the Valley of the Aire, Yorkshire.^ The width, depth, 

 and coarseness of the fluviatile deposits of the Mississippi are, in 

 proportion to an average of sevenfold the rainfall and nearly 

 twofold the velocity. 



On the 4th of October, 1858, the current was 1-72 feet per second, 

 and the water held in suspension 138 grains of solid matter per 

 cubic foot of water, or 22-13 grains per gallon. In the Pluvial 

 Period the author supposes the water held in suspension 160 grains 

 per gallon in _i._ of the weight of water. 



Messrs. Humphreys and Abbot state (p. 399, op. cit.) that at New 

 Orleans, thirty -nine feet below the surface, the marine^ strata begin, 

 or those belonging to an earlier geological age than the present, or, 

 at least, before the material brought down by the Mississippi river 

 as now existing began to accumulate in this locality, 



I have written to Dr. Bendict, at New Orleans, for the names of the 

 shells, but have as yet received no answer. Sir C. Lyell records the oc- 

 currence of an estuarine shelP near Lake Pontchartrain at a great 

 depth, and very near New Orleans, where, on the contrary, Humphreys 

 and Abbot describe marine shells. This requires expanation, as well 

 as many of their other opinions as to the age of the clay imme- 



1 See p. 63, Quart. Journ., 1869. 



2 The probability of the Mississippi Delta being partly marine was suggested by 

 A. T., p. 272, Phil. Mag., 1853 : — " If the further examination of the Delta of the 

 Mississippi shows marine or fluvio-marine strata," etc. 



3 Gnathodon cuneatus, a common brackish-water bivalye. 



