In the West, where volcanic materials are abundantly distributed among the rocks of the geologic column, the importance of sedimentary processes in the formation of high-alumina clays has not been fully appreciated. At Ione, Calif., Castle Rock, Wash., Whiteware, Mont., Hobart Butte and Mollala, Oreg., where the Geological Survey has been investigating clays jointly with the Bureau of Mines, U. S. Department of the Interior, high-alumina clays formed by sedimentary processes are more important than those formed by all phases of volcanic activity. Clays derived directly from volcanic materials are composed dominantly of montmorillonite; but these clays, with the exception of the beidellite-nontronite varieties, have relatively low percentages of available alumina. In contrast, clays derived by thorough leaching of various aluminous rocks under conditions favoring thorough drainage are composed dominantly of kaolinite and when sorted by sedimentary processes form high-grade deposits; furthermore, the depositional structures of these sedimentary clays have favored the derivation of gibbsite through weathering, resulting in an increase in the alumina content of the clays. Kaolinite, gibbsite, and dickite all have relatively high percentages of available alumina.