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



phenogams because of the dense shade, are comparatively rich in 

 confcrvae. 



It is sufficiently evident that there is nothing in tropical condi- 

 tions which would prevent the accumulation of peat. Where there 

 is a long dry season, the vegetable matter is exposed as is that in 

 the ordinary upland forest in the temperate regions. The accumu- 

 lation of the humus cover advances rapidly for a time but at length 

 the waste by oxidation about balances the additions, so that the thick- 

 ness does not increase, though the trees shed a greater quantity each 

 year. But in a lowland area where the moisture is great, the chem- 

 ical changes are modified, the loss is diminished and increased supply 

 brings about increased accumulation. Swamps arise, when from any 

 cause the drainage is impeded. Even along the flow of small springs, 

 peat forms when the water is held back from any cause. On exten- 

 sive areas, such as the coastal plain on the Atlantic side of the 

 United States or the delta of a river like the Mississippi, where at 

 best the drainage is imperfect, the streams being sluggish and often 

 serpentine, the drainage has been hindered still further by vegeta- 

 tion, the moist area was enlarged and swamps of vast extent origi- 

 nated in post-glacial times. The important condition is the constant 

 supply of water ; the drainage must be impeded on the surface 

 and through the bed. In the northern part of the United States 

 probably the greater part of the swamps rest on an impervious bed 

 of glacial clay, an underclay ; but it is not necessary that the imme- 

 diately underlying bed be of normally impervious material, for many 

 large swamps have a floor of fine sand. 



Harper"- penetrated 10 or 12 miles into the Okefinokee swamp 

 of southern Georgia and discovered that the material on which the 

 peat rests is a few feet of Columbia sand overlying the clay, loam 

 or coarse sand of the Grand Gulf formation (Lower Miocene or 

 Upper Oligocenc). In some places a "hard pan," colored by vege- 

 table matter and cemented by iron underlies the sand. Sanford'^ 



"R. M. Harper, "Okefinokee Swamp," Pop. Sci. Month., Vol. LXXIV., 

 TQog, p. 596. 



" S. Sanford. "Topography and Geology of Southern Florida," 26. Ann. 

 Rep. Gcol. Survey of Florida, 1909, P- I93- 



170 



