Big Hill, Matagorda, is a characteristic Gulf Coast salt dome and has a distinct salt-dome mound, a subcircular salt core intruded into Tertiary sediments and a thick cap on the top of the salt. The cap contains an extensive and important deposit of sulphur. The cap is composed of (a) a "barren cap" of limestone above, (b) the sulphur-bearing zone composed chiefly of limestone and grading below into (c) a thick deposit of barren anhydrite. The sulphur is secondary and later than the limestone and anhydrite. The sulphur is mined by the Frasch process in which superheated water is pumped down through a drilled well; the melted sulphur is collected at the bottom of the well and pumped to the surface by an air lift. A review is given of the possible reactions for the formatio of the sulphur. On account of the lack of extensive drilling, little is known about flanks of the dome, a small amount of oil was produced in the early days, mostly from the top of the cap.