Potassium-argon dates indicate an Eocene age for six Tertiary floras that represent vegetation zones as diverse as mixed deciduous hardwood forest, conifer-hardwood forest, and subalpine forest. Paleobotanical age criteria suggest these upland floras are much younger, confirming evidence that indicates Eocene forests that lived in regions of temperate climate had the aspect of Neogene floras. Certain potassium-argon dates preeviously assigned to Tertiary floras disagree with paleobotanical evidence for their age. Some of the discrepancies reflect the problem of altitude and the floras of younger aspect that lived there, but others appear to be due to dating rocks not associated with the floras, to contaminated samples, and to other reasons.