The unique Hector bentonite occurs along a northwest-trending fault zone for a distance of about five miles. Hot springs emanating along this fault during Pliocene (?) time deposited travertine in a then existing lake in which sand, marl and silicic pyroclastics also were being deposited. The tuff lying on the travertine benches was altered to saponite by solutions containing lithium and fluorine in the last stages of hot spring activity, with the magnesium being extracted from the lake water. Analcime, representing alteration of the tuff when the springs were inactive, is found below and above the bentonite bed. Clinoptilolite is present directly associated with the hot spring activity.The amount of travertine decreases to the northwest as do the contents of lithium, fluorine, and magnesium in the clay, which approaches an aluminum montmorillonite in composition. In a restricted basin where magnesium is available, acid solutions from hot springs apparently produce a chemical system that forms a trioctahedral bentonite preferentially to a dioctahedral variety even though the parent material approximates the chemical composition of an aluminum montmorillonite.Minor faulting and slumping have produced gouge zones and small downward displacements of the irregular bentonite lenses on the southwest side of the main fault. The deposit is located in a valley of low relief; the dominant structural feature is the high, developed by the deposition of sediments and pyroclastics on the travertine benches. River gravels and muds, and a basalt flow in places, unconformably overlie the series.