PartIIL] OR POISONED BY VEGETABLE GROWTH? 
209 
covered with forests from many different causes; 
but the last cause I should assign would be 
the soil becoming exhausted for trees. The form¬ 
ation of bogs by the over-luxuriance of woods 
may be one of these causes. I think it possible 
that, in some cases, the irruption of peat into 
woods from the bursting of bogs above them 
may have violently overthrown the trees; in 
this case the trees should all lie down the 
stream: or that the drift from above, and quiet 
deposit of alluvial peat into woods, by suffocat¬ 
ing the roots, may have killed the trees and 
caused their fall, instead of the fall of the trees 
causing the bog; but in this case it is unlikely 
that the stems should lie all one way, yet this 
is possible if the tract is only exposed to wind 
on one side. 
p 
