558 STEVENSON— FORMATION OF COAL BEDS. [November 3, 



which rise 3 to 10 feet above the water and give off much inflam- 

 mable gas. They were studied by Sidell'*- for the Mississippi survey 

 of 1838 and by Humphreys and Abbot at a later time. Sidell be- 

 lieved that in outflow of the river the finest materials, organic matter 

 and silt, went farthest and, after deposition, were covered with 

 coarser materials. Decomposition of the organic matter generates 

 gas, which lifts the overlying deposits. According to Humphreys 

 and Abbot, the mud lumps are formed on the crests of bars and 

 their activity ceases when the gas is exhausted. In 1858, during 

 operations for removal of obstructions, some mud lumps on the bar 

 of Southwest pass were broken by an explosion of gunpowder. 

 Strong ebullition of gas continued over a wide area for 20 minutes 

 after the explosion and the surface of the bar, in a space, 100 feet 

 diameter, sank, assuming the form of an extinct crater. Hilgard*" 

 gives as composition of gas from one of the mud lumps, marsh gas, 

 86.20, carbon dioxide, 9.41, and nitrogen, 4.39, which closely resem- 

 bles the average composition of gas from swamps. Oxygen is absent. 

 Very little of the vegetable material is stranded on the banks of 

 rivers, comparatively little is deposited on the deltas. Most of the 

 great accunuilations in the Mississippi delta, supposed at one time to 

 be driftwood, have proved to be buried forests in place. Terrace 

 deposits along the Monongahela, in Pennsylvania and West Virginia, 

 contain only here and there a woody fragment in the mass of sand, 

 clay, gravel and blocks of rock. The same is true of terrace deposits 

 generally, away from the lines of abandoned curves. In this con- 

 nection. Brown's"" observations along the Amazon are instructive. 

 Long lines of cliffs, now on one side, now on the other, are composed 

 of bright-colored deposits, contrasting with the monotonous clay 

 banks of the river. The elevated plateau of these old river deposits, 

 as well as the alluvial plain, is covered with luxuriant forest. Ex- 

 cept a narrow strip along the bank, the whole plain is overflowed 



"Humphreys and Abbot, op. cit., pp. 485, 486; W. H. Sidell, Ibid., Ap- 

 pendix A, p. 499. 



^" E. W. Hilgard, "The Exceptional Nature of the Mississippi Delta," 

 Science, N. S., Vol. XXIV., 1906, p. 865. 



°'' C. B. Brown, " On the Ancient River Deposit of the Amazon," Q. J. 

 Geol. Soc, Vol. XXXV., 1879, pp. 763-777. 



151) 



