Abstract A core of rock salt containing potassium salts and fossil algae was recently taken, at a depth of 4,800 feet, from Gray No. 1 well of the Rycade Oil Corporation at the Markham, Texas, salt dome. This is the first occurrence of potassium salts reported from the salt domes of the United States, and is probably the most important contribution of fact to a study of their origin for a score of years. The first appreciable step toward a knowledge of the real structure of the domes was the discovery of the main salt mass of Petite Anse, one of the Five Islands of Louisiana, by the deepening of an old brine well in 1862. We came to know the general form, composition, and structure of our domes as a result of the vigorous drilling campaign which followed close upon the discovery of an oil pool on the Spindletop dome in 1901. The active exploration of the domes since that time, in the mining of oil, sulphur, and salt, has served chiefly to emphasize the essential regularity of the domes, their general conformability to type, and to emphasize the uplift of the overlying and contiguous strata by the formation of the salt core and cap rock of the dome. The first theories of origin—little more than vague speculations based on entirely inadequate conceptions of the true nature of the domes—regarded them as old Cretaceous outliers in Tertiary seas. With a fairly satisfactory working knowledge of the true constitution of the

Abstract A critical review of American thought on salt-dome origin shows that from the discovery of American salt domes in 1862 until the establishment of their economic importance by the development of a cap-rock pool at Spindletop in 1901, little was known of the constitution of the domes except what little was expressed at the surface. The result was a wide variety of highly speculative theories of origin, chief of which, as best fitting our meager knowledge, was the theory that the domes were old Cretaceous islands in Tertiary and even Recent seas. Exploration of the known domes for cap-rock pools and sulphur deposits from 1901 to 1916–18 made us better acquainted with the salt, anhydrite, gypsum, limestone, sulphur, and various minor minerals—the salt-dome materials—and theories of deposition from solution became the vogue. The development of important oil deposits in the lateral sands flanking the salt masses, from 1912 to date, concentrated attention on the structural features of the dome. The deposition-from-solution and lifting-power-of-crystallization theories seemed to be inadequate to explain the sharp and considerable uplift caused by the formation of the salt core and cap rock, and, with a growing recognition of the similarity of American salt domes to the salt structures of Germany, Roumania, Mexico, and elsewhere, came a gradual swing to the theory of tectonic origin. This theory of tectonic origin supposes that the plastic salt was forced by pressure to flow from originally bedded deposits into its present position. The author accepts the tectonic or pressure-flowage theory for the origin of American domes and believes that the most serious objection raised by its opponents—the lack of evidence as to the existence of sedimentary salt deposits—is no longer tenable in view of the recent discovery of potash salts and fossil algae in the salt core of the Markham dome.