140 
AGE CHARACTERISTICS OF COALS 
deposits off the coasts of Holland, Belgium and northern France 
are continuous with living bogs on the mainland; but the buried 
peat, in greatest part, is older than that now exposed, as evidently 
the march crept gradually inland during the subsidence. In like 
manner, the great deposits formed on plains show notable varia¬ 
tion in thickness as well as in composition. The vast peat-overed 
plains of Alaska and Siberia have contemporaneous top-layer, 
but the underlying portions of the deposits are probably very far 
from being strictly contemporaneous. 
The condition prerequisite to formation of peat is an abundant 
supply of moisture, with sluggish drainage; this does not mean that 
alternating wet and dry seasons are necessarily preventive. If 
the supply of moisture suffice to keep the main mass moist the 
loss during dry season is more than made good by growth during 
the wet season, as shown by some tropical swamps. This condi¬ 
tion of moisture depends greatly upon the topography which 
determines the character and extent of drainage. In the cold 
regions decomposition is less advanced than in lower latitudes and . 
the accumulation is of vegetable matter rather than of peat prop¬ 
erly so-called. The fact is certain that in the tropics as in the 
temperates peat accumulates where the necessary conditions exist, 
and that it does not accumulate in either when those conditions 
are wanting. 
Peat may be derived from any land plant; but ordinarily the 
flora contains many types. The constituent plants vary at the 
several horizons in a deposit. For the most part the peat does not 
consist of any one plant or class of plants. Occasionally a layer 
consists of a single species; but this occurrence is relatively rare. 
The peat-making forms are not the same in all localities. In 
northern Europe, and also in some parts of North America, 
certain mosses are the important constituent in the upper layers; 
but there are considerable areas in both regions where mosses 
are either wanting or are wholly unimportant. Sedges have been 
the efficient peat-producers in much of the north temperate, and 
even in some tropical and subtropical localities. But there is no 
limitation; conifers, palms, deciduous trees, mosses, sedges, in 
a word, any water-loving plant or any plant preferring a slightly 
acid soil will yield peat under similar conditions, and the soft 
parts soon become pulp but the harder parts change more slowly. 
The felted structure of the peat is not due to any special char- 
