THE CHEMISTRY OF SOILS AND MANUEES. 
1. One purely mechanical agency in the formation of soils is the force of gravitation. In the case 
of overhanging or steep precipitous rocks, when one or all of the chemical causes already mentioned 
have acted long enough to weaken the cohesive force which keeps the mass in its position, so that 
the force of gravity preponderates over that force, the part so influenced tumbles down in obedience 
to the law of gravitation, and contributes to fill up the valley below with the disintegrated fragments. 
The stony masses, in their passage downwards, are reduced to smaller pieces, small fragments and 
light powder all aggregated together. According to the nature of the rock, vegetation springs up on 
these debris more or less luxuriantly, often very rapidly. When visiting, some years ago, the scene 
of the fall of the Rossberg on the village of Goldau, near the Rigi, in Switzerland, which took place 
in 1806, I found the greater part of the debris covered over with a luxuriant vegetation. 
2. The fine powder resulting from the disintegration of rocks, and fine sand, are easily moved by 
heavy winds, when they are found covering the ground in unsheltered places. Heavy winds con- 
tribute much in changing the aspect of some localities, particularly those near the sea-coasts. On the 
shores of the Baltic large tracts of arable soil are annually covered with drift-sand. On the south- 
west coast of France the dunes increase every year about seventy feet in breadth. The effects of the 
wind on the largest scale are exhibited in the deserts of Africa ; many of the colossal antiquities of 
Egypt are almost completely covered with sand. 
3. Water, which acts, as we have seen, chemically in dissolving some of the constituents of rocks 
more readily than others, exercises likewise a most powerful influence in changing the nature of rocks 
in a purely mechanical way, thus contributing to the formation of new soils. The rain-water which is 
absorbed by different rocks freezes in winter, and expanding in becoming ice, bursts the rock into many 
fragments more thoroughly than almost any other mechanical contrivance, in the same way as we see 
a good winter frost after a fall of rain, pulverising the soil better than any clod-crusher can do. 
Further, a powerful influence which water exercises in breaking up rock masses and degrading emi- 
nences to lower levels, is due to what is called the hydrostatic paradox. Any one who is at all 
acquainted with the laws of hydrostatics will at once admit this as a most powerful disintegrating 
force. Again, the rains continually wash off the fine particles from the decomposed higher rocks, 
and cany them down to lower levels ; and on occasions of great floods not only the finer particles, 
but gravel, larger fragments of minerals, and even heavy stones are transported to great distances, 
and distributed often in large quantities over extensive districts of a country. Brooks and rivers 
carry along with them marks of the country through which they flow. In the stages of their move- 
ment the coarser materials are first deposited, the finer particles and mud move along with the stream, 
and are finally deposited more or less completely at the mouth of the rivers, where the flow of the 
waters gradually becomes slower at periods of comparative rest. These deposits form the alluvial 
soils of our river banks. The vast mass of materials deposited at the mouth of large rivers, such as 
the Mississippi, the Indus, the Amazon, the Rhine, alters the condition of the soils along the banks of 
the deltas of these rivers from a naturally sterile into a most rich and fertile one. 
The quantity of matter which a river thus brings down varies with the length of its course, the 
velocity of its current, and the nature of the rocky beds through which it passes in its course towards 
the sea. According to Mr. Horner, a thousand gallons of the waters of the Rhine near Bonn hold in 
suspension two-thirds of a pound of mud, which for the greater part is deposited near the mouths of 
the river in the Netherlands. It is by this sediment that the low banks of the Rhine are annually 
fertilised, and rendered capable of producing luxuriant crops of flax. Ln Holland, many originally 
sterile, sandy soils have been rendered most productive by such alluvial deposits. 
4. The sea likewise plays an active part in changing the characters of the land near the shore, and 
in giving rise to new soils. Whilst, on the one hand, the constant motion of the waters is continually 
encroaching on the land in one place, washing away and carrying along with its waters the sands of 
the shore and the debris of the rocks at the sea-coasts, it deposits them partly again in other places 
on more sheltered shores. 
From some of the materials which the sea-water contains in solution, many animals of the lower 
orders build up their skeleton and shells. Oyster-banks and coral-reefs are thus produced below the 
surface of the sea-level ; gradually they rise above the water, forming often an extensive island, on 
which at first but few plants spring up. These die and manure the soil with their remains ; a third layer 
of a more fertile soil is thus produced, on which a greater variety and number of plants may vegetate. In 
the course of time the decay of every succeeding generation adds to the mass of the soil, and the originally 
sterile coral-reef gradually becomes more and more capable of supporting a healthy vegetation. 
5. Vegetable remains, to which we have already alladed, and especially animal remains, contribute 
more to the formation of some soils than might probably be expected. The waters of the river, but 
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