The Houston Salt Company is sinking a shaft for the mining of salt in the north-central part of the Hockley salt dome,3 near the northeast corner of the Coghill Survey, northwestern Harris County, Texas. This operation is of particular interest, not only because the porous water-bearing calcite and limestone zone at the top of the cap series has been successfully passed through and the shaft is now perfectly dry, but also because of the opportunity it has offered to study, at first hand, the cap-rock section and the cap-rock-salt contact (Fig. 1), and because of the data which strongly suggest a sedimentary origin for the anhydrite.
The calcite or limestone part of the cap is brecciated and interwoven with veins of calcite and flecked with calcite vugs, many of which hold oil. Minor amounts of barite and strontianite occur ordinarily as concretions. At the base of the limestone is a steel-gray calcitic sand 15–30 inches thick. This sand is in sharp contact with the smooth, slightly polished upper surface of the gypsum. The gypsum is 18 feet thick and grades into the anhydrite through a vertical distance of 3 feet. The gypsum-anhydrite contact dips northward feet in the 15-foot width of the shaft. (See shaft section, Fig. 4.)