STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 33 



they concern matters to be considered on another page. He was 

 surprised by the abundance of floating and aquatic plants. Already, 

 within the few weeks of the lake's existence, great beds of water 

 lettuce and of water hyacinth had covered much of the surface and 

 were associated with extensive patches of green scum. The water 

 hyacinth had become a pest. The destruction of forests had been 

 rapid ; non-water-loving trees were killed by gradual rise of the 

 water-level, but palms showed great power of endurance. Fre- 

 quently one of the latter is seen with its trunk completely submerged 

 and only the crown of leaves showing above, resembling a bunch 

 of gigantic ferns on the surface. 



But when dried by exposure to light and air peat is unstable 

 material. Change in direction of drainage may deprive a consider- 

 able area of the needed moisture and growth will be stopped. 

 Unless the surface be invaded by trees, running water will break 

 continuity of the bog and that will be ruptured into " hags," of 

 which Scottish writers have given vivid descriptions. In a moist 

 climate, this process of destruction is slow, as is seen in much of 

 England and Scotland, because peat absorbs moisture and retains it 

 with great tenacity. It may well be that growth may be checked or 

 wholly stopped in one portion of an area while it continues in 

 another, as in the Fenland of England, where peat still grows in 

 one district, though the general climatic change has caused cessa- 

 tion elsewhere. If untoward conditions continue, the peaty cover 

 becomes desiccated and is removed by the wind or other agencies — 

 a fact which is unpleasantly familiar to those who have cultivated 

 drained peat bogs. 



Pcat-foniiing Plants. — Peat, being merely vegetable material 

 undergoing chemical change with greater or less exclusion of 

 oxygen, may be the product of any group of plants. The popular 

 belief, based on surface study of bogs in- northern Europe, has 

 always been that mosses are the chief source of material for peat; 

 and this no doubt led to the conception that no true peat is to be 

 found within the tropics, since neither Sphagnum nor Hypmtm 

 prospers amid tropical conditions. Yet more than lOO years ago 



