English: Sulfur (yellow) and gypsum (gray) forming a cap rock of salt dome. Sulfur is a product of bacterial activity. Salt dome contains anhydrite (calcium sulfate), especially in the upper parts (cap rock) because salt (NaCl) is more soluble and gets preferentially carried away by groundwater. Anhydrite becomes gypsum when hydrated. Salt domes are often associated with hydrocarbons (crude oil and natural gas) because salt domes are ideal structural traps which restrict the rise of hydrocarbons. These hydrocarbons are source of energy for bacteria who decompose anhydrite to form hydrogen sulfide, calcium carbonate and water: anhydrite (CaSO4) + CH4 (hydrocarbons) + bacteria = H2S (hydrogen sulfide) + limestone (CaCO3) + water. H2S reacts with oxygen to form elemental sulfur and water: 2H2S + O2 = 2S + 2H2O. So, this rock is a perfect illustration how the cap rock of a salt dome looks like which also contains crude oil or natural gas. The width of the specimen is 11 cm. Additional information from the source: [2]