sentences:  It has a higher content of magnesium carbonate, strontium carbonate, calcium sulfate (gypsum) and silicon dioxide(silica) than is found in typical limy sediments elsewhere.It occurs as a superficial calcareous crust no more than a few millimetres thick, which is generally white, grey, or brownish with a pearly lustre.It was believed to be formed in the intertidal zone by saltwater spray and evaporation, but an algal contribution has recently been suggested (Montanari and others (2007)), following an overlooked earlier proposal by E. Onorato in 1926.