1149. Alluvial soil which has been deposited by rivers is considered the most fertile, as it generally consists of minute particles of soil of various kinds mixed with salts and other minerals, and containing animal and vegetable matters in a state of complete decay. It is now well known that those soils are most fertile which contain the greatest number of different ingredients ; and as alluvial soil must have been gathered by the rivers which deposit it from many different lands, and as its particles must have been in a state of minute subdivision to be held in solution by the water, they must, of course, have been intimately mixed, and this is probably the cause of the great fertility of soils of this description. It must be observed, however, that it is only sluggish rivers which deposit rich alluvial soil, and that rapid currents are exceedingly injurious to the lands they overflow. The effect produced naturally by rivers is sometimes imitated by irrigation.