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agents bring about disintegration, among them wind, 
flowing water, and moving ice with their abrasive 
action; thermal contraction and expansion resulting in 
cracking; and frost with its wedging action. Some of 
these agents also effect most decomposition, tin* atmos- 
phere, water itself, and aqueous solutions being 
chemically reactive. The products of weathering vary 
considerably in such properties as grain size, density, 
and solubility in water and their subsequent history is 
dependent in part on these properties. 
Trn asportation. 
Transportation is brought .about, through the agencies 
of wind, water, and ice. Material is carried mechan- 
ically and in solution and while in motion is often 
subjected to further disintegration and decomposition. 
Erosion is the term applied to the beginning of transpor- 
tation. When the velocity of a stream of water or air is 
reduced, so is its transporting power and some of its load 
is dropped- Substances in solution are precipitated as 
a result of evaporation or by chemical reaction. The most 
rapid deposition of sediments in quantity occurs in the 
sea on the continental shelf, in the lower parts of the 
courses of rivers and in lakes. Particles laid down by 
water or wind tend to be sorted according to weight and 
size. This results in their deposition in layers and the 
terms bedding, stratification and lamination are applied 
to this structure. Thus one many see in a river denosit 
a bed of mud superimposed on a bed of sand. It follows 
that, the thickness of any bed of sediment depends on 
the length of time that deposition continues in that place. 
Consolidation. 
The deposited sediment may be consolidated and a 
sedimentary rock produced. This consolidation generally 
takes place long after the material has been buried under 
other sediments. It is brought about by compaction and 
cementation. Compaction, due to the weight of over- 
lying sediments, results in the loss of water and the 
particles are brought closer together and may interlock. 
Cementation involves the deposition from solution of 
adherent substances such as hydrated iron oxide, 
calcium carbonate, and silica, in the pore spaces of the 
sediments. 
The complexity of all these sedimentary processes is 
increased by the part played by organic materials. 
Animals and plants often supply much of the source 
