Filter media are to a large extent chemically inert materials of fine fibrous or granular structure, which develop filter layers of high permeability. The filter bed consists mostly of diatomaceous earth (diatomite or kieselguhr), which is manufactured from fossil remains of diatoms, a type of algae having a silica skeleton. A number of other media may be employed for precoated filters. Chief of these is perlite, which is processed from a vitreous rock of organic origin. Other precoat media include powered organic rock, activated carbon, asbestos and cellulose