iQii] STEVENSON— FORMATION OF COAL BEDS. 557 



packed deposit, was merely enough to load a Danish merchantman 

 of one hundred years ago — say a vessel of 400 or 500 tons. The 

 description shows also that the wood had floated long ; it matters not 

 whether it came by the slow drift from Siberia or Norway to north- 

 ern Greenland and thence southward, or followed the north coast 

 eastward to Davis strait, it is certain that the voyage was very long 

 and the wood showed the efifects. This long continued buoyancy 

 of the wood is equally evident in the Gulf of Mexico, where one 

 finds the Mississippi drift wood stranded on the shores along hun- 

 dreds of miles. 



In all extensive deposits of driftwood, the trees are battered, 

 stripped of leaves, bark and often of branches; they are scattered 

 on the strand or piled in irregular loose heaps, where, exposed to 

 the air, they decay; or if in more favorable conditions, the inter- 

 stices become filled with sediment, the trees become merely logs in 

 shale or sandstone, even their genus being unrecognizable except by 

 microscopic study of the structure. 



The quantity of finely divided organic matter transported by 

 rivers is minute in comparison with that of inorganic. Pourtales^^ 

 examined sediment from samples of Mississippi water collected at 

 Carrollton, Louisiana. The first series, taken in March during a 

 flood from the Red and Ohio rivers, yielded no matter of organic 

 origin aside from some spicules of sponges and rare vegetable fibers. 

 A second series, collected in June, during a Missouri flood giving the 

 most abundant sediment of the year, contained in water from the 

 surface some indistinct vegetable fibers and wood cells, but no re- 

 mains of vegetables were found in the other samples taken at various 

 depths. A third series, in August, contained a vegetable scale or a 

 leaf of moss; while a fourth series, in October, contained no organic 

 matter. A fifth series, collected in the following January, showed 

 on nearly all the filters minute black bodies which may have been 

 pollen or spores. 



But the absolute quantity of organic matter carried out is con- 

 siderable. The " mud lumps " ofi^ the mouth of the Mississippi, are 

 masses of tough clay, occasionally forming islands of several acres, 



■" L. F. Pourtales, in Humphreys and Abbot, Appendix 8, p. 651. 



155 



