Chalk — sedimentary rock of white, soft and friable, insoluble in water, organic (zoogenic) origin. The basis of the chemical composition of chalk is calcium carbonate with a small amount of magnesium carbonate, but usually is present and non-carbonate part of mainly metal oxides. The chalk is usually a slight admixture of fine grains of quartz and microscopic calcite pseudomorphs on fossil marine organisms (Radiolaria, etc.) Often found large fossils of the Cretaceous period: belemnites, Ammonites, etc.