The Washakie, a volcanic ash formation. Bulletin of the AMNH ; v. 26, article 4.

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New York : Published by order of the Trustees, American Museum of Natural History
"The difference in petrographic character between the Bridger and Washakie has an important bearing on the problem of correlation in time. The Bridger rocks are rhyolite tuffs containing glassy sanidine while the Washakie rocks are andesitic with soda-lime feldspar. From the absence of agglomerates and the fine-grained character of much of the ash it seems probable that it was transported mainly by the wind, and as the prevailing winds are at present from the west and had probably the same direction in Tertiary time, the centers of eruption should be located somewhere in the west or southwest. The absence of agglomerates does not favor the idea of local contemporaneous vents discharging rhyolitic and andesitic ash respectively and the great thickness and uniform petrographic character of each formation is opposed to the conception of rapid variation in the chemical composition of the ash at a single center of eruption. Assuming contemporaneous deposition from two centers of eruption it seems probable, in view of the comparatively short distance separating the areas occupied by the two formations (about 50 miles)¹ that some intermixture of the two types of ash should be found, but the conspicuous absence of plagioclase feldspar from all the Bridger tuffs and its presence in all those of the Washakie shows that this has not occurred. The lithologic evidence, therefore, does not favor the idea of contemporaneity for any part of the Bridger or Washakie. The faunistic evidence of contemporaneity of the lower Washakie and Upper Bridger is therefore in contradiction to the lithological evidence. ¹Estimate based on map of Fourth Parallel Survey"--P. 26-27.
p. 25-27 ; 24 cm.
Includes bibliographical references.