412 ELIOT BLACKWELDER 
In the opinion of the author, there is a significant relation 
between the position of this dissected plateau and the bodies of 
Eocene sediments in the north Rockies and plains adjacent on the 
east. The inference is made that these sediments could not have 
been produced by the dissection of the plateau after it was elevated, 
partly because the volume of material obtainable from such dissec- 
tion would be supposedly insufficient, and partly because the 
drainage seems to have been westward rather than eastward, since 
the uplift. 
In the first place, it seems evident that no sufficient quantitative 
study has ever been made of the volume of either the Eocene 
sediments or the material removed in dissecting the plateau, to 
give the first argument any considerable weight, especially as we do 
not know to what extent other regions to the north, east, or south 
may have contributed sediments. As to the second point also, it 
may be said that no connection has been shown between the Eocene 
strata and the source of the sediments, and that it seems within 
the bounds of probability that material may have come from 
several other directions as well as from the west. In this connec- 
tion I may point out that Mr. Umpleby’s map, showing the distri- 
bution of sediments of Eocene age which he thinks may have been 
derived from the peneplain, includes large outcrops of the Fort 
Union and correlative formations. Yet the Fort Union in the north 
Rockies has been upturned, folded, and beveled off, and upon its 
trunkated edges the Lower Eocene strata were subsequently depos- 
ited. It would therefore seem necessary to believe that the Fort 
Union formation was deposited before the completion of the deforma- 
tive movements which the author rightly thinks preceded the cycle 
of erosion represented by the peneplain. 
For these reasons I can see but little value in the train of argu- 
ment by which the author reaches the conclusion that the peneplain 
furnished the material for the Eocene sediments and which leads 
him to say: ‘‘That the plateau surface is of Eocene age, there 
seems to be little room for doubt.” In view of the fact that several 
interpretations other than those suggested by the author may be 
applied to the observed data, it seems to me that there is very 
large room for doubt. There are, indeed, some additional facts 
