64 THE farmers' handbook. 



minerals expand and contract unequally when subject to variations in tem- 

 perature, and their alternate contraction and expansion is an important 

 factor in the slow breaking down of rocks. 



The daily alteration of temperature between night and day is capable of 

 effecting the disintegration of the hardest rocks. This disintegration is 

 accelerated by the fact that into the minute crevices so formed, water, dust, 

 plant-roots, &c, are carried, themselves acting as further disintegrating 

 agents. 



On mountain ranges and over extensive plains, where frosts occur, and the 

 air is clear and dry, the action is yet more marked. The surface soil in the 

 deserts of Central Asia and other parts of the world, including the interior 

 of our own continent, has been formed in this way, aided, of course, by the 

 other weathering agencies. 



Water. 



The effect of water, and especially of water containing carbonic acid in 

 solution, as is the case with rain-water and most spring and river water, has 

 a most far-reaching influence in the disintegration of rocks and their conver- 

 sion into soil. 



(a) Mechanical Action of Water. — Water acts mechanically in several 

 ways. The actual bombardment of exposed surfaces of rock by rain results 

 in its slow attrition. Running water traversing rock surfaces exerts a very 

 considerable erosive action apart from its solvent action. The action is 

 further hastened by the removal to lower levels of the particles thus torn 

 away and the exposure of fresh surfaces of rock to the weather. 



Running water also plays an important part in the transport of soils from 

 one part to another, from higher to lower levels, and in the formation of 

 alluvial soils by the deposition of mud and fine silt during flood. Such 

 alluvial soils occur along the banks of all our rivers for some distance from 

 the river banks, and in the case of rivers which wind tortuously through 

 level country and are subject to frequent floodings the extent of the alluvial 

 deposit is very considerable. When such a river, as is the case with the 

 Hunter, frequently changes its course, the extent of the alluvial deposit 

 becomes still greater. 



The alternate freezing and melting of water plays an important part in 

 mechanical rock disintegration. Even the most compact rocks are not 

 entirely impervious to water, and contain a small amount of moisture, 

 known to quarrymen as " quarry water." Water in freezing expands about 

 one-tenth of its bulk, so that in situations exposed to alternate frosts and 

 thaws, the amount of disintegration brought about by this means is very 

 considerable, particularly when the alterations are rapid, say, warm days 

 followed by frosty nights. During periods of prolonged frost this action 

 does not proceed. 



Connected with the action of moving water, we must also take into account 

 the action of glaciers, which, though it takes no present part in the formation 

 of soils on the Australian continent, has undoubtedly been an active agent 

 in the past in the disintegration of mountain ranges and the transport of the 

 debris to form soils in the valleys. 



(b) Chemical Action of Water. — The chemical action of water, and 

 especially of the dissolved substances, is undoubtedly the most extensive 

 agent in the denudation of rocks. 



Water acts on the soil constituents in two ways — 



(a) By combining chemically with the ingredients to form hydrates; 

 and (b) by its solvent action. 



