AN OLD EROSION SURFACE IN IDAHO 145 
The value of this surface in time determinations is perhaps 
greatest in dating the ore deposits of the plateau region. Two 
distinct periods of mineralization are recognized in this area. The 
earlier deposits are cut by the Eocene surface, but the later are 
inclosed in or associated with eruptive rocks which fill valleys 
developed after its elevation. Thus the Eocene surface was 
developed during the interval between two great periods of 
mineralization. Reasoning from it as a datum plane the deposits 
may be rather definitely placed in time. On the one hand is the 
Pleistocene glaciation and the amount of erosion which preceded 
it but followed the veins, thus placing a fairly definite limit. On 
the other the granite, which is older than the earlier veins, is 
assigned to the latest Cretaceous or earliest Eocene as brought 
out in the next section. 
AGE OF GRANITIC INTRUSIONS SUGGESTED BY THE 
EOCENE SURFACE 
There are many granitic batholiths within the plateau region. 
The largest of these is the one in central Idaho, which is more than 
20,000 square miles in extent. Several others approach to or 
exceed 1,000 square miles in area, and those of smaller size are 
to be numbered by the score. Indeed, probably one-third of 
the surface rock throughout the present plateau region is granite 
or closely allied batholithic types. Both broadly and_ locally 
these intrusions vary from normal granite through soda granite 
and quartz-monzonite to quartz-diorite. Their distribution is 
shown by Fig. 3. A comparison of this with Fig. 2 brings out the 
striking accordance in distribution between the area of granitic 
intrusions and the plateau. Their coextent suggests a genetic 
relation between them, but the granite constitutes a larger part 
of the plateau surface; hence, if a relation exists it must date from 
the earlier part of the erosion cycle during which the Eocene 
surface was developed. The problem, therefore, is to show whether 
the granite entered at this or at a still earlier time. Probably 
most geologists will agree in the opinion that such a tremendous 
volume of magmatic material did not enter beneath the region 
without causing or accompanying a profound elevation of the 
