AGE CHARACTERISTICS OF COALS 
141 
acter of the plants, for it is present in forest litter. The extent 
of chemical and physical change increases downward in a deposit. 
At the top of a growing bog one finds living plants; but within 
two or three inches the mass consists of dead material, slightly 
changed in color, but with small increase in percentage of carbon. 
Lower down the organic structures become less and less dis¬ 
tinct, and at length the whole mass is, to the unaided eye, merely 
pulp, in which are embedded fragments of wood and occasional 
leaves. 
Stages of growth in the peat deposits depend very largely upon 
the character of the original topography of the area. In the 
filling of water-basins the first stage is the formation of mud on the 
bottom. The accumulation of peat has been continuous in few 
localities; even small deposits show pauses like those which char¬ 
acterize those of great extent. Many times a cyclical order 
is distinct and the deposit is divided into benches. The benches 
may pass gradually, the one into the other, or they may define 
sharply by partings. At times the partings may consist of mineral 
charcoal mingled with extremely fine mineral matter, the residuum 
on the surface of the peat long exposed to oxidation. Such 
partings mark a period of dryness without invasion by forest 
during which the peat wasted. But partings of clay, sand and 
marl mark invasions of water carrying detritus. 
Expansion of peat deposits by transgression has been observed 
in all parts of the world. In many deposits of wide extent the 
fact of transgression becomes evident only after removal of the 
peat for fuel during reclamation. The effect of pressure on peat 
is to render it so similar physically to brown coal that the re¬ 
semblance to the latter is very strong. 
Peat contains introduced materials of various kinds. Logs and 
stumps of trees are not of this class; they are merely the more 
resistant parts of peat-making plants. Fragments of rock, some¬ 
times angular, sometimes water-worn, have been reported from 
some localities. The infrequency of references may indicate rarity 
of occurrence, localization within the peat, or the indifference of 
observers. The facts available are so few that any suggestion 
as to origin of these fragments would be worthless. Often there 
is much silt; at times one finds pockets of sand or clay, and even 
fresh-water limestone. 
The several branches of a peat deposit often differ notably 
