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Parr Il. Szcr. ii. § 1.] GROWTH OF SOIL. 
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: Formation of Soil.—On level surfaces of rock the weathered 
erust may remain with comparatively little rearrangement until 
plants take root on it, and by their decay supply organic matter to 
the decomposed layer, which eventually becomes what we term 
“veoetable soil.” Animals also furnish a smaller proportion of 
organic ingredients. Though the character of soil depends primarily 
‘on the nature of the rock out of which it has been formed, its fertility 
‘arises in no small measure from the commingling of decayed 
animal and vegetable matter with decomposed rock. 
A gradation may be traced from the soil downwards into what 
is termed the “ subsoil,” and thence into the solid rock underneath. 
Between soil and subsoil a marked difference in colour is often 
observable, the former being yellow or brown, when the latter is 
blue, grey, red, or other colour of the rock beneath. This contrast, 
evidently due to the oxidation and hydration especially of the 
iron, extends downwards as far as the subsoil is opened up by 
rootlets and fibres to the ready descent of rain-water. ‘The yellowing 
of the subsoil may even occasionally be noticed around some stray 
rootlet which has struck down further than the rest, below the 
general lower limit of the soil (postea, Section iii.). 
Mr. Darwin observed many years, ago that a layer of soil three 
inches in depth had grown above a layer of burnt marl spread over 
the land fifteen years previously; also that in another example a 
_ similar layer had, as it were, sunk beneath the soil to a depth of 
twelve or thirteen inches in eighty years. He connected these facts 
with the work of the common earth-worm, and concluded that the 
fine loam which had grown above these original superficial layers 
had been carried up to the surface, and voided there in the familiar 
form of worm-castings." This action of the earth-worm is doubtless 
highly important, but, as Richthofen has pointed out, we have to 
take also into account that gradual augmentation of level due to the 
- daily deposit of dust (ante p. 321), 
Soil being composed mainly of inorganic, and toa slight extent of 


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Fie. 92.—SEcTION SHOWING THE UPWARD PASSAGE OF Rock (a) InTO Sussorn (0) 
AND THENCE INTO VEGETABLE SOIL (e). 
organic materials, the proportion between these two elements is a 
question of high economic importance. With regard to the organic 
matter, it is the experience of practical agriculturists in Britain that 
oats and rye will grow upon a soil with 13 per cent. of organic 
_1 Geol. Trans. v. 1840, p. 505. 
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