Our lab's recent analysis of archaeological obsidian from the Zuni and Hopi areas suggested that, unlike the somewhat vitrophyric glass from Grants Ridge, prehistoric knappers preferred an aphyric glass that while chemically similar, does not elementally covary with samples from Grants Ridge. Systematic survey and sampling this summer resulted in the discovery of another source on Horace Mesa to the east of East Grants Ridge. These nodules up to 8 cm in diameter are aphyric and are a better medium for tool production. The chemistry differs in a number of incompatibles, but appears to be derived from the same magma source of high silica rhyolite, a late Tertiary and early eruptive phase in the Mount Floyd field. Selected topographical and geological map of study area.
Contours are approximate, but accurate at points noted.
All measurements in meters. Analytical methods and instrument settings in Shackley 1998 . This is a single analysis by WXRF of a fused disk standard. Analytical methods and instrument settings in future publication. Zr, Y, Nb three dimensional plot of Horace Mesa and Grants Ridge source standards. .