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rock is presented to the action of the air. Here we have one cause of the change 
of the form of the earth’s surface. The above may be said to be the chief action 
of the air. 
We will now look at the next agent—wind. The wind, as everybody knows, 
is only the air in motion, but the effects and action of wind are sufliciently 
recognisable to strike the mind of most people and to convince them that the wind 
is a very important agent. Its effects as a soil-forming agent are best seen in hot 
dry countries, such as North Africa, where the soil is very friable, parched, and 
light : in such countries the wind carries the sand and dust along with it for miles, 
and often deposits it far away from its parent rock. Although in our island we © 
do not see the effects of wind on the soil on such a large scale, yet any one who has 
ever visited a sandy sea shore must have witnessed considerable hillocks or sand- 
dunes, as they are called, formed by blown sand. The oxidising power of the air 
is of course in action in the wind as well as in still air, so that the wind has a 
double action, both chemical and physical. Lastly, I will only just mention the 
power of the wind to waft seeds and other vegetable matter to all parts of the 
earth, where, if the circumstances are suitable for them, they take root and grow 
up, and thereby bind together the loose particles of the soil by their roots. This 
may be called an indirect effect of the wind in the formation of soils. 
T must now pass on to rain. Rain washes down all loose and friable matter, 
and carries it on till it deposits it at the bottom of some valley or river-bed ; it 
also finds its way through the interstices of rocks, and by its chemical action as 
well as its physical, takes up in solution some of the substance of the rock through 
which it passes ; this kind of action I will speak of when treating of the effects of 
water. 
* 
Snow like rain washes down loose materials, but it is large masses of accu- 
mulated snow such as are seen in Switzerland and elsewhere that have such a 
powerful effect in breaking off huge masses of rock and bearing them away to be 
disintegrated by the air and other forces. 
Compact frozen snow, in the form of glaciers, wear away, and erode the 
valleys through which they pass, and carry away with them an immense amount 
of materials which are deposited when the snow andice melt and form a ‘‘terminal 
moraine.” 
Frost is another important agent. Frost is the name applied to that state 
of atmosphere which is below 32 {degrees on Fah. thermometer, or below zero on 
the centigrade ; its most obvious effect is that water congeals and becomes ice: 
this is a most important effect as water when it becomes ice increases in bulk, and 
of course becomes specifically lighter; so that when water which has run into 
crevices of rocks freezes, it expands and cracks the rock and frequently breaks off 
large masses, just as gun-powder would do; these masses of rock either roll down 
the hill side and at the bottom gradually decay and form soil, or they are borne 
away on the crest of a glacier or ice berg far from their parent rock. This affords 
an explanation of the frequent phenomena of boulder-stones, which are stones 
es 
