492 A. Ti/Ior — Formation of Deltas. 



diately below the modern alluvium. Sir C. Lyell's account of the 

 stratification of the Delta appears much more precise than that of 

 Messrs. Humphreys and Abbot, who seem to wish to bring the forma- 

 tion of the whole of the alluvium of the Mississippi within a period 

 of 4,400 years. As far as I can ascertain, this view of the Delta, 

 280 miles long, 70 miles wide, having been formed in 4,400 years, is 

 entirely contrary to their own evidence. 



Mr. Sidell (p. 10, Appendix A, op. cit.) writes respecting the 

 lumps which rise on the river and sea-bottom near the passes of the 

 Mississippi into the Gulf of Mexico. These lumps have a peculiar 

 appearance. In entering the mouth of the river they may be taken 

 for rocks, from the steepness of their sides, their compactness, and 

 the appearance of stratification produced by cracks. In some cases 

 they rise eight or ten feet, nearly perpendicularly, and at one place 

 there is a mound in the shape of a truncated cone, ascertained to be 

 at least eighteen feet high. It is nearly inaccessible through the 

 marsh or flat that surrounds it. 



The material of the lumps is a very fine clay; it decrepitates with 

 heat. On many of these lumps are found springs of salt water. 

 The spring issues through a well-defined crater as firm on the sides 

 as a chimney, generally of about six inches diameter. There is an 

 ebullition of the water of the spring at considerable intervals, and 

 inflammable gas constantly escapes from some of them. This is 

 probably light carburetted hydrogen gas. The salt water stands in 

 these springs two or three feet higher than the level of fresh water 

 of the surrounding river. Some of these lumps form islands several 

 acres in extent, they are only found in the immediate vicinity of the 

 Gulf, and they form the nucleus for shoals. They are reduced by 

 violent rain, and are supposed to be raised by pressure of gas formed 

 below the surface by the decomposition of vegetable matter, the 

 hollow being filled up with mud. The surface of many lumps is 

 covered with white pure salt, evaporated from the deposit of the 

 springs (see p. 16, Appendix A). 



The origin of these lumps is thought to be chemical rather than 

 mechanical, and they evidently differ from mounds formed in marsh 

 ground which have been raised by tipping ballast near it. 



It is recorded that during the Survey of the Mississippi bar a 

 lump two feet high was discovered in the Channel which had arisen 

 within two or three hours. 



On the more probable hypothesis that 3 parts in ] 00 of the whole 

 material denuded are deposited on the Delta, and not removed 

 again, and that the remaining 97 parts are carried out to sea before 

 being deposited, then the extension of the whole Delta, 55 miles (by 

 means of fluviatile deposit alone), into the Gulf of Mexico would 

 require sufficient time to effect the denudation of over 2,455,250 

 square miles of surface, averaging 100 feet. 



I calculated the present rate of denudation of this area in 1853, 

 and my figures have been recently adopted by Mr. Croll. I found 

 that the denudation was going on at the rate of 1 foot in 10,000 years 

 in the above area, and at this rate we should require 10,000 x 100, 



