546 STEVEXSOX— THE FORMATIOX OF COAL BEDS. r^ov. i. 



samples consisted of small gravel and coarse sand with very little 

 clay. No coarse material is carried beyond New Orleans but 

 pebbles occur in the lower alluvium near that city and the bars at 

 the mouths of the passes show very distinctly the effect of sorting- 

 The source of the coarse material was not determined, but in any 

 event it is certain that the gravels had travelled hundreds of miles.^*^ 



The story has been continuous along the Mississippi since early 

 in the Tertiary. The river has made its valley in the soft rock.s 

 forming its bluffs, where successive deposits are exposed. In the 

 later Tertiary as well as in the alluvial deposits, one sees the cypress 

 swamps which at one time were at the surface. The erect stumps 

 and fallen trunks are present, the condition being wholly like that 

 in recent swamps, except that the trees are dead. Forested and 

 buried swamps are numerous and of great extent on the alluvial 

 plain below Red River, but their area has been diminished by drain- 

 age and the protection afforded by levees. The buried swamps, sup- 

 posed at one time to be composed of drifted vegetable matter, are 

 known now to be in situ. 



It is not always easy to draw the line between delta deposits and 

 those made on flood plains, as is evident from Medlicott's observa- 

 tions on the Indo-Gangetic region. Belt's^*' studies in a portion of 

 the Siberian plain may be taken as complementary to those by 

 Medlicott, for they show how widely coarse material may be dis- 

 tributed by rivers. Leaving Ekaterinburg, he reached the level, 

 sandv region of the steppe within 60 miles and continued his journey 

 in east-southeast direction to Ischim, on a tributary of the Irtisch, 

 where the steppe wall, 80 feet high, is composed of sand without 

 pebbles and partly cross-bedded. Thence to Omsk on the Irtisch, 

 the plain is monotonous but at that city he saw a section showing 

 60 feet of the steppe deposit, in which the sand, at times cross- 

 bedded, contained lines of pebbles, none larger than a cherry, with 

 here and there broken shells of Cyrena fluminalis. At Omsk, he 



^*° A. A. Humphreys and H. L. Abbot, " Physics and HydrauHcs of the 

 Mississippi River," 1876. pp. 45, 90, 92, 147, 673. 



'"•T. Belt, "The Steppes of Siberia," Qiiari. Joiirn. Geo!. Soc, Vol. 

 XXX., 1874, pp. 490-498. 



