“ It signifies the gradual accretion of land or formation of an island by imperceptible degrees. If the accretion or formation be by a torrent or flood, the property in the severed portion or new island continues with the original owner until the trees, if any, swept away with it take root in the ground. Alluvion never attached at all in the case of agri limitati, that is, lands belonging to the state and leased or sold in plots. ”