A rapid electron probe technique has been developed for characterising pyroclastic deposits by the composition of their voleanic glass. Samples of Mazama ash (6600 years old) from northwestern USA and southwestern Canada were used to test the compositional variability of the clear unaltered glass shards. The electron probe analyses show that the glass composition is remarkably consistent, even though some sample locations are several hundred miles apart. The glass composition of Glacier Peak ash (about 12,000 years old), St. Helens 'Y' ash (about 3,000 years old) und Bridge River ash (about 2,500 years old) were also determined and found to be distinctive. Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Cl and F were determined to investigate the usefulness of each in characterising these ashes. It was found that as few as three key elements, for example, Ca, K and Fe or Ca, K and Na, serve to identify any one of these ashes.