The volcanic ash or dust in the Great Plains region was first recognized by Wadsworth, who in 1884 described deposits of such material that lie between the White and Niobrara rivers.3 Four years earlier Aughey4 erroneously identified similar material as geyserite, and that name persisted in the literature for several years and is still used locally as one of the many synonyms for volcanic ash. In 1896 Cragin5 described some volcanic ash found in Meade County, Kansas, which he named Pearlette ash, on the assumption that it formed a well defined stratigraphic unit. Pearlette was the name of a near-by post office, now abandoned. Other early papers on these deposits were written by Merrill6 and by Barbour.7
Within a few years after the discovery and recognition of volcanic ash on the Great Plains it became known that deposits of this material occurred over a . . .