230 JOSEPH B: UMPLEBY 
several belts of lava in east-central Idaho all occupy valleys devel- 
oped by streams of the same river system. Of these several beds, 
which from their topographic relations may reasonably be thought 
to be of about the same age, all have been referred to the Miocene 
except the Payette formation. In view of this fact it may be 
pertinent to review briefly our knowledge of the age of the Payette 
formation. 
The Payette formation was first assigned to the Miocene,’ but 
later because of a change in the reference of the deposits of Bridge 
Creek, Ore., which, in addition to a somewhat similar flora, have 
yielded vertebrate remains, it was shifted to the Eocene.’ 
The correlation of the Payette formation with the deposits of 
Bridge Creek is based on six species of plants which they have in 
common. The Payette flora, however, ‘embraces 32 forms, of 
which 17 were described as new and 5 were not specifically named, 
leaving, as then known [the first report] only 1o species having an 
outside distribution.” Five of these were found at Bridge Creek 
‘‘and to this list I am now able to add another species, thus making 
6 of the 10 species common to these two localities.” 
The other lake beds of the plateau region have not as yet 
yielded nearly as varied a flora as the Payette but it is hoped that 
additional search in the near future will add greatly to the collec- 
tions from them. It is possible that during the next field season 
one of the paleobotanists of the United States Geological Survey 
will devote considerable time to this problem. 
The only change which such studies may make in the age of the 
old erosion surface as now defined is to crowd it back toward or into 
the Cretaceous. This would result if the several localities yielded 
definite Eocene plant remains. ‘The principal reason for my belief 
that such a change will not be made is expressed in my first paper 
as follows: 
It seems that the [surrounding Eocene] sediments could not have been 
derived from the region after its last elevation for two reasons: (1) It is very 
doubtful if the plateau is sufficiently dissected to afford the volume of material 
1 Waldemar Lindgren, Twentieth Ann. Rept. U.S. Geol. Survey, Pt. III (1900), 95. 
2F. H. Knowlton, Bull. U.S. Geol. Survey No. 204 (1902), p. IIo. 
3 [bid., p. 110. 
