Depositional structures of chemical and biological origin

Much of the material weathered and eroded from land areas is transported to the seas as ions in solution. From geochemical studies, it is known that the composition of sea water has remained fairly constant throughout a large part of geological time and it follows that ions must have been taken out of solution by the precipitation of new minerals. This precipitation can be inorganic or it can be aided by, or due entirely, to organic agencies. The most abundant minerals precipitated from sea water are aragonite and calcite, and most of this precipitation results from microbes, plants or animals.