Algae absorb sea water containing calcium and magnesium carbonate and other particles, which eventually coagulate and precipitate to form a type of fossil called a stromatolite. Alternating laminated layers of different colours form as result of seasonal changes and varying rates of deposition. Stromatolite layers can have different shapes, including laminar, spherical, hemispherical, columnar, cylindrical, conical and dendritic. Stromatolites are abundant in China. They are mined in the Proterozoic dolomite and dolomitic limestone strata in the north and the Sinian dolomitic limestone and siliceous dolomite strata in the south.