Recent work has resulted in major changes in the stratigraphic correlations and source-area assignments of ash-flow sheets of the late Miocene Black Mountain and Stonewall Mountain volcanic centers, southern Nevada. Ash-flow tuffs exposed around Stonewall Mountain have different trace element patterns and thermoremanent magnetization directions than do ash-flow sheets of the Thirsty Canyon Tuff, erupted from the Black Mountain center, with which they were previously correlated. Hafnium concentrations are particularly useful in distinguishing the megascopically and petrographically similar tuffs of the two centers. The ash-flow sheet overlying the spearhead Member in the Stonewall Mountain area, herein named the Civet Cat Canyon Member, can be traced northward continously from outflow facies south of Stonewall Mountain to near-vent facies on the southern flank of Stonewall Mountain. This, in conjunection with distribution, thickness, and facies relations show that these units are the outflow sheets of the Stonewall Mountain volcanic center; they are herein assigned to a new formation, the Stonewall Flat Tuff of late Miocene age. Tuffs previously mapped near Black Mountain as the Labyrinth Canyon Member (herein abandoned) are correlated on the basis of trace-element, paleomagnetic, petrographic, and radiometric age data with the e Spearhead Member of Goldfield. Tuffs previously assigned to the Spearhead Member in the Black Mountain area are reassigned to the Pahute Mesa Member (new) of the Thirsty Canyon Tuff. The Stonewall Mountain volcanic center is thus slightly younger than the Black Mountain center, and the period of time over which the Black Mountain center was active probably was appreciably shorter than was previously inferred. Recognition of the Stonewall Flat Tuff as a separate stratigraphic and genetic entity enables us to explain a number of geological inconsistencies, including the marked asymmetric distribution previously attributed to the Thirsty Canyon Tuff is significantly reduced, increasing the importance of lava relative to tuff in the eruptive history of the Black Mountain center.