1911.] STEVENSON— FORMATION OF COAL BEDS. 529 



But ill January, February and IMarch, when the absorbed moisture 

 in the htter was great, the contrast still remained, as appears from 

 the second column, where the run-off from the forested areas aver- 

 ages only three eighths while that from the non-forested area was 

 nineteen twentieths. 



The great dunes of Bermuda have their advance checked by vege- 

 tation. A network of vines creeps over the surface and breaks the 

 force of the wind. Clumps of grass take root in the open spaces 

 and, within a brief period, the heavy rains can do little more than 

 to move the sand a few inches to be piled against the obstacles. 

 Vegetation holds its place on the loose materials until, at length, a 

 dense growth of oleander and cedar render the deluge-like rains 

 wholly ineffective. The same condition exists along railroads within 

 the drift covered areas of the United States. ]\Iany of the through 

 cuts are in drift gravels, with no trace of consolidation, yet their 

 walls show the steady advance of plants in spite of rain and the 

 steep slope. 



The resistance which vegetation offers to erosion is manifest on 

 a grand scale in the tropics, where growth is luxuriant and the rain- 

 fall extreme. The writer has had opportunity to examine at close 

 range fully 200 miles of the Venezuelan and Colombian coast, much 

 of Trinidad, about 50 miles of the Jamaican coast, as well as much 

 of the Pacific coast of Central America. There are some localities 

 where the rock is not consolidated and vegetation cannot maintain 

 itself. Such as gains rooting toward the close of the wet season is 

 killed during the dry season and rain finds only the unprotected sur- 

 face on which to act. But such areas are of limited extent. The 

 slopes along the coast are usually quite steep and the stratified rocks 

 commonly dip at a high angle. Landslides, owing to this structure, 

 are not rare and they leave a scar on the surface which persists for 

 years ; but aside from those merely temporary interruptions, vege- 

 tation is practically continuous on even the steepest slopes. The 

 Jamaican conditions are especially instructive. Where vegetation 

 was destroyed by fire in some extensive areas, Guinea grass has 

 taken possession of even the steepest slopes, giving great spaces of 

 bright green, which are notable features of the scenery — and this in 



127 



