Abstract The period 1905 to 1980 brought many changes in mining law, land policy, mineral taxation, trade policy, and environmental regulation in the United States. In mining law, the chief changes affected mineral development on the public domain. The claim system of the Mining Law of 1872 was supplemented and partly replaced by a leasing system, an assertion of the doctrine of sovereign rights. Despite changes, both claim and leasing systems were aimed at providing access to the public domain for purposes of mineral development. Land policy, however, placed increasing restrictions on access. By the end of 1979, large areas of the public domain were closed or effectively closed to mineral exploration and development, and additional withdrawals and restrictions were under consideration. In 1905, taxation on the American mining industry was relatively light. Beginning in the 1910s, however, tax measures proliferated, and levels of taxation increased. By 1980, the burden of taxation had become a significant factor in decisions regarding mineral exploration and development. United States trade policy affecting domestic mineral development varied greatly from 1905 to 1934. After 1934, however, trade policy made an effort to achieve the freest possible international trade. Deterioration of the mineral position of the United States became accepted as the price of providing mineral raw materials to the consumer at the lowest possible cost. In view of growing deficiencies in mineral supply from domestic sources, stockpiles of minerals were created to provide protection from interruptions in the flow of minerals from abroad. On occasion, stockpiles were used in efforts to influence mineral markets. A growing concern over the impact of mining and mineral use on the environment and on public health and safety led in the 1960s and 1970s to increasingly stringent regulation. Great strides were made in protecting the environment, but the measures taken involved substantial costs to the American mining industry. As the 1970s closed, the nation was still struggling to achieve a suitable balance between its needs for environmental protection and its needs for mineral raw materials. The changes cited above reflect changing social attitudes toward development of the mineral resources of the United States. The cumulative effect has been substantial alteration of the social and political framework within which mineral development must take place.

Abstract The chromite deposits of the eastern Bushveld Complex occur in the Critical Zone, a layered pile consisting mainly of pyroxenite, norite, and anorthosite, with minor gabbro, dunite, and harzburgite. With probable reserves in excess of one billion tons, the deposits are among the major mineral resources of the world. The Critical Zone is more than 75 miles long and is divisible into several sectors. The central and southern sectors are best known. Field studies indicate that they differ in rock sequences and interlayered chromite deposits. An 11,000 foot sequence of rocks in the central sector, extending from the Basal Zone upward into the Main Norite Zone, has been sampled in detail, and its evolution is being studied in terms of field data, mineralogy, chemical composition, and textures. The sequence is divisible into successive units on the basis of silicate mineralogy. The Critical Zone is also divisible into chromitic and non-chromitic intervals, according to the presence or absence of cumulate chromite. Within the chromitic intervals occur the chromitites, in which chromite is the sole cumulate. Most chromitic intervals consist of chromitic pyroxenite or anorthosite, or both, but two consist mainly of chromitic dunite and harzburgite. Broad mineralogical and compositional trends—upward change from predominant pyroxenite to predominant norite and anorthosite, upward increase in total Fe, Fe/Mg, and Fe/Cr of chromite, in Fe/Mg of orthopyroxene, and in Ab/An of plagioclase—suggest that the Critical Zone is grossly the result of progressive magmatic differentiation. Departures from these trends, however, together with major disconformities in the sequence, indicate a complex history. This is reflected in the distribution and characteristics of chromitites.