Samples of North American obsidian were irradiated for 40 sec. in a pneumatic tube facility of the University of Michigan's twomegawatt nuclear reactor. Errors arising from the neutron irradiation procedure were minimized by interrupting the irradiation and shaking the capsules contg. the samples in order to redistribute the samples. The samples were counted by a 2 x 2-in., well-type NaI(Tl) scintillation detector and a single-channel analyzer covering the energy range 0.76 to 0.93 MeV. A computer program was used to resolve the data for each sample and standard into its ²⁴Na and ⁵⁶Mn components by the method of least squares. The Na and Mn contents of over 1900 obsidian samples were detd.; the range in compn, of these two elements for a geologic source is not more than 1.35. The Na content may differ by more than 1. 35. The Na content may differ by more than a factor of two and the Mn by a factor of ten or more for different geologic sources. -- AATA