lime stone
Limestone often contains variable amounts of silica in the form of chert (aka chalcedony, flint, jasper, etc) or siliceous skeletal fragment (sponge spicules, diatoms, radiolarins), as well as varying amounts of clay, silt and sand sized terrestrial detritus carried in by rivers. The primary source of the calcite in limestone is most commonly marine organisms. These organisms secrete shells made of aragonite or calcite and leave these shells behind after the organism dies. Some of these organisms can contruct mounds of rock known as reefs, building upon past generations. Below about 3,000 meters, water pressure and temperature causes the dissolution of calcite to increase non-linearly so that limestone typically does not form in deeper waters (see lysocline). Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves). This produces speleothems such as stalagmites and stalactites. Another form taken by calcite is that of oolites (oolitic limestone) which can be recognized by its granular appearance.