Abstract: In the intermountain states, a region deficient in kaolinite and alumina clays, there is an increasing demand for refractory clay. In this arid region residual kaolins are not developed in quantity from feldspathic intrusives. Glassy, siliceous extrusives alter commonly to montmorillonite of the montmorillonite-beidellite series. Zeolites, principally analcime and heulandite, often are alteration products. Under certain conditions kaolinite, halloysite, or saponite will form; several tuffaceous deposits were investigated in an attempt to determine these environmental conditions, especially the physico-chemical environment necessary to produce kaolin clay. Zeolites are developed where the pyroclastics are deposited in an alkaline lake; saponite is formed locally in this environment as a result of hot-spring activity. On river slopes and spurs the siliceous volcanics alter to kaolinite and montmorillonite; where associated with calcareous hot-spring activity in a fresh-water lake, halloysite results. Montmorillonitebeidellite develops where these special conditions do not obtain. The prospects of finding a residual kaolinite deposit developed from siliceous volcanics are not good. Hydrothermal alteration appears to be the only means by which a siliceous volcanic will be converted to a sizeable refractory clay deposit in this region.