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458 DYNAMICAL GEOLOGY. — [Boox III. _ 
increase, and likewise by themselves form not unimportant deposits. 
Of plant formations the following illustrative examples may be 
iven :— 
: 1. Humus, Black Soils, &c—Long continued growth and 
decay of vegetation upon a land surface, not only promotes disinte- 
gration of the superficial rock, but produces an organic residue, the 
intermingling of which with mineral débris constitutes vegetable 
soil. Undisturbed through long ages, this process has, under fayvour- 
able conditions, given rise to thick accumulations of a rich dark 
loam. Such are the “regur,” or rich black cotton soil of India, the 
“tchernayzem,” or black earth, of Russia, containing from 6 to 10 
per cent. of organic matter, and the deep fertile soil of the American 
prairies and savannahs. These formations cover plains many 
thousands of square miles in extent. The “tundras” of northern 
latitudes are frozen plains of which the surface is covered with arctic 
mosses and other plants.’ 
2. Peat-mosses and Bogs.—In temperate and arctic 
latitudes, marshy vegetation accumulates in places to a depth of 
sometimes 40 or 50 feet in what are termed bogs or peat-mosses. 
In northern Europe and America these vegetable deposits have been 
largely formed by mosses, especially species of sphagnum, which, 
growing on hill tops, slopes, and valley bottoms as a wet spongy 
tibrous mass, die in their lower parts and send out new fibres above. 
Among the Alps, as also in the northern parts of South America, 
and among the Chatham Islands, east of New Zealand, the same part 
is played by various phanerogamous plants, which form on the surface 
a thick stratum of peat. A succession can sometimes be detected 
in the vegetation out of which the peat has been formed. Thus in 
Europe among the bottom layers traces of rush (Juncus), sedge (Iris), 
and fescue-grass (estuca) may be observed, while not infrequently 
an underlying layer of fresh-water marl, full of mouldering shells of 
Limnea, Planorbis and other lacustrine molluscs, shows that the area 
was originally a lake which has been filled up with vegetation. 
The next and chief layer of the peat will usually be found to con- 
sist mainly of matted fibres of different mosses, particularly Sphagnum, 
Polytrichum, and Bryum, mingled with roots of coarse grasses and 
aquatic plants. The higher layers frequently abound in the remains 
of heaths. Every stage in the formation of peat may be observed 
in the section cut in mosses for fuel: the portions at the bottom being 
more or less compact dark brown or black, with comparatively little 
external appearance of vegetable structure, while those at the top 
are loose, spongy, and fibrous, where the living and dead parts of the 
mosses commingle (Fig. 171). 
It frequently happens that remains of trees occur in peat-mosses. 
1 It may be well to take note here again of the extensive accumulation of red 
loam in limestone regions which have long been exposed to atmospheric influences. 
To what extent vegetation may co-operate in the production of this loam has not been 
determined. Fuchs belicves that the “terra rossa” is only present in dry climates 
where the amount of humus is small. (Ante, p. 338, and authorities there cited.) 
