Abstract: Large hydrothermal deposits of halloysite clay occur in the Terraced Hills, Washoe County, Nevada and similar bodies probably are present elsewhere in the Basin and Range province. The host rock, an andesitic tuff, is underlain and overlain by volcanic flows; all these rocks are late Miocene to Pliocene in age. The clay bodies are composed mostly of halloysite with some iron oxides, variable amounts of feldspar and quartz, and locally some montmorillonite. Commonly all the pyroclastic unit is altered to halloysite material. In one locality, however, the halloysite body is restricted to the upper part and it is in sharp contact with underlying, partly montmorillonitized tuff. The contact of a clay body with the overlying basalt is distinct. Generally some halloysite is present in the lower part of the basalt and montmorillonite occurs in both materials near their contact. The solutions that altered the tuff were generated during volcanism, rose along high-angle faults, and were restricted to the permeable and otherwise favorable vitric tuff by the capping of relatively impermeable basalt.