Two cinder cones and associated basaltic lava flows near Williams, Arizona, contain numerous inclusions of ultramafic and of felsic material. Many of the felsic inclusions are semifused and contain high 87Sr/86Sr ratios, yet the enclosing basalts do not show any enrichment in 87Sr when compared with other local basalts. Brown glass within a quartz monzonite inclusion has a much higher 87Sr/86Sr ratio than the inclusion as a whole. This is believed to reflect the domination of early-forming melts by biotite. If so, separation and assimilation of this glass by a magma offers a mechanism to increase the 87Sr/86Sr ratio of that melt without significant bulk assimilation of granitic material.