30 
FORMATION OF BOGS. 
They generally originate in tlie checking of watercourses by 
the falling of timber, or of earth and rocks, across their chan¬ 
nels. If the impediment thus created is sufficient to retain a 
permanent accumulation of water behind it, the trees whose 
roots are overflowed soon perish, and then by their fall 
increase the obstruction, and, of course, occasion a still wider 
spread of the stagnating stream. This process goes on until 
the water finds a new outlet, at a higher level, not liable to 
similar interruption. The fallen trees not completely covered 
by water are soon overgrown with mosses; aquatic and semi- 
aquatic plants propagate themselves, and spread until they 
more or less completely fill up the space occupied by the 
water, and the surface is gradually converted from a pond to a 
quaking morass.* The morass is slowly solidified by vegetable 
either the area or the depth of the deposit. In any event, however, hogs 
cover hut a small percentage of the territory in any of the Northern States, 
while it is said that one tenth of the whole surface of Ireland is composed 
of bogs, and there are still extensive tracts of undrained marsh in England. 
Bogs, independently of their importance in geology as explaining the 
origin of some kinds of mineral coal, have a present value as repositories 
of fuel. Peat beds have sometimes a thickness of ten or twelve yards, or 
even more. A depth of ten yards would give 48,000 cuhio yards to the 
acre. The greatest quantity of firewood yielded by the forests of New 
England to the acre is 100 cords solid measure, or 474 cubic yards; but 
this comprises only the trunks and larger branches. If we add the small 
branches and twigs, it is possible that 600 cubic yards might, in some cases, 
be cut on an acre. This is only one eightieth part of the quantity of 
peat sometimes found on the same area. It is true that a yard of peat and 
a yard of wood are not the equivalents of each other, but the fuel on an 
acre of deep peat is worth much more than that on an acre of the best 
woodland. Besides this, wood is perishable, and the quantity on an acre 
cannot be increased beyond the amount just stated; peat is indestructible, 
and the beds are always growing. 
* “ Aquatic plants have a utility in raising the level of marshy grounds, 
which renders them very valuable, and may well be called a geological 
function. * * * 
“ The engineer drains ponds at a great expense by lowering the surface 
of the water; nature attains the same end, gratuitously, by raising the 
level of the soil without depressing that of the water; but she proceeds 
more slowly. There are, in the Landes, marshes where this natural filling 
