ARCHEAN AND PALEOZOIC GEOGRAPHY 461 
The vegetation which by its life and death made 
possible the coal beds that we now find so useful, fell 
in such a position that it was protected from decay in 
the air; and after a certain accumulation had been 
made, was buried beneath layers of sediment, and hence 
still further protected. Plants that die and drop upon 
the ground, soon decay and return the greater part of 
their substance to the air and earth; but in swamps 
this rapid disintegration is checked by the presence of 
water and preservative acids, that are generated by the 
decaying plants. Hence in swamps it is possible for 
vegetation to accumulate in thick beds, as we may see 
in any peat bog of the northeastern states (see p. 190). 
The fact that coal-forming plants were able to accu- 
mulate in considerable thickness, proves that when they 
fell they dropped into bodies of water. Whether these 
coal marshes were actually salt or fresh water swamps 
cannot now be stated. At present but one kind of tree, 
the mangrove (p. 251), is able to grow with its roots in 
salt water. This tree, where it exists in tropical lands, 
forms jungles quite like those which many believe to 
have existed during the Carboniferous times. It is by 
no means impossible that the primitive trees of the 
Carboniferous were able to exist when surrounded by 
salt water, although the more advanced trees of the 
present would be destroyed by the presence of salt 
water about their roots. 
