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 Panr IT. Suov.i.§1.] GROWTH OF DUST. 321 
exposed at once to great drought and to rapid vicissitudes of tem- 
perature, a continuous lowering of the general level takes place. 
- The great sandy wastes thus produced represent, however, only a 
portion of the disintegration. Vast quantities of the finer dust are 
_borne away by the wind into other regions, where, as will be im- 
mediately pointed out, they tend to raise the general level. 
Again, a considerable amount of fine dust and sand, blown into the 
neighbouring rivers, is carried down in their waters. In inland 
areas of drainage, indeed, like that of Central Asia, this transport 
does not finally remove the river-borne sediment from the basin 
of evaporation, but tends to fill up the lakes. Where, however, as 
in North America, rivers cross from the desert areas to the sea, 
there must be a permanent removal of wind-swept detritus by 
these streams. In the arid plateaux drained by the Colorado and its 
tributaries, so great has been the subaerial denudation that a thick- 
ness of thousands of feet of horizontal strata has been removed from 
the surface of level plains thousands of square miles in extent. This 
denudation, the extent of which is attested by the remaining cliffs 
and “ buttes”’ or outliers of the strata, appears to be in great measure 
due to the causes here discussed, augmented in some districts by 
the effects of occasional heavy storms of rain. 
One further effect produced by air in violent motion may be 
- seen where, in forest-covered tracts of temperate latitudes, trees are 
occasionally prostrated over considerable spaces. The surface 
drainage being thus obstructed by the fallen stems, marsh plants 
spring up, and eventually the site of the forest is occupied by a 
peat-moss. (Section iii., Life.) 
2. Reproductive action.—Growth of Dust. The fine dust 
and sand resulting from the general superficial disintegration of 
rocks would, if left undisturbed, accumulate in situ as a layer that 
would serve to protect the still undecayed portions underneath. 
Such a layer, indeed, partially remains, but being liable to continual 
attack and removal, may be taken to represent, where it occurs, 
the excess of disintegration over removal. In the vast majority of 
eases, however, the superficial coating of loose material is not due 
merely to the direct action of the air, but in far greater degree to the 
work of rain aided by the co-operation of plants and animals. To 
the layer thus variously produced, the name of Soil is given. Its 
formation is described at p. 339. : 
| That wind plays an effective part in the re-distribution of 
superficial detritus is demonstrated by every cloud of dust blown 
from desiccated ground. We only need to take into account the 
_ multiplying power of time, to realize how extensively the soil of a 
district may be replenished and heightened by the dust thus strewn 
over it century after century. Dust and sand intercepted by the 
leaves of plants gradually descend to the soil below or are washed 
down by rain, so that even a permanently grassy surface may be 
slowly and imperceptibly heightened in this way. 
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