The absolute ages of three major chronological divisions of Tertiary volcanic activity and accompanying mineralization in the Marysvale, Utah, area are given on the basis of potassium-argon age determinations.

These results agree closely with the geological relationships and the age for mineralization derived by the uranium-lead method. Unaltered glass, biotite, sanidine, and sericite give reliable results, but argillic alteration in glass produces low apparent ages owing to loss of radiogenic argon.

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