A Study in Rock-Weathering

Four districts furnished samples for a study in rock-weathering. Mineralogical data and fourteen new chemical analyses are presented in tables and diagrams. The granite gneiss of Morton, Minnesota, shows a marked loss of soda and lime in early stages of weathering but a more gradual loss of potash and baria. The progress of the decomposition is interpreted from a series of six samples of residual clay. Kaolinite is an end product. New chemical analyses of material from the Medford, Massachusetts, diabase are presented as a check on the older data. Ferrous and ferric iron determinations on samples of diabase and of glacial till suggest that the till is less oxidized than the underlying diabase, indicating preglacial weathering of the diabase. A diabase on the north shore of Lake Superior, Minnesota, is weathered locally to a depth of 40 feet but shows little chemical change except oxidation. An amphibolite from the Black Hills, South Dakota, yielded beidellite or related clay minerals by decomposition of its hornblende. Calcite has been leached, and other minerals attacked. On the basis of the present and earlier studies, a mineral-stability series in weathering is proposed. The arrangement of the common rock-forming minerals in this series is found to coincide with the reaction series. Experimental work on the attack of silicate minerals by water is in accord with the suggested mineral-stability series.