414 ELIOT BLACKWELDER 
have been shown by Willist and Smith and Calkins? to be notably 
folded into a series of well-marked anticlines and synclines. There 
seems to be adequate proof in the Snoqualmie quadrangle that this 
deformation took place about the middle of the Miocene period, so 
that the case is relieved of the usual difficulties arising from uncer- 
tainties of correlation. If the peneplain is Eocene in age it must 
have suffered the same deformation that produced in these Eocene 
strata average dips of to and in many places of more than 25°. 
Under those circumstances it could not retain so nearly its original 
plane character that (barring intrenched valleys) it still differs in 
altitude only 5,000 feet in 400 miles, or in other words, declivities 
of but a small fraction of one degree. Rather, the evidence seems 
to show that this peneplain is much younger than Eocene and 
probably post-middle-Miocene. 
The comments here made upon Mr. Umpleby’s article have 
been called forth by the fact that the peneplain, definitely assigned 
to the Eocene, is offered to students of Rocky Mountain geology 
as a valuable “datum plane in broad areas where time relations 
between the Algonkian and the Pleistocene are otherwise obscured.” 
The peneplain probably has real existence, and may be used as a 
datum plane to determine the relative ages of other geologic events 
in the same region; but it should not be regarded as an index of 
Eocene age, and its exact chronological value will depend upon a 
more reliable future determination. 
G.O.Smith and B. Willis, ‘Geology of Central Washington,” U.S. Geol. Survey, 
Prof. Paper 19, 1904. / 
2G. O. SmitH and F. C. Carxkins, ‘‘ Reconnaissance of the Cascade Range,” 
U.S. Geol. Survey, Bull. 235, 1904. Also Snoqualmie, Wash., Folio (139), U.S. Geol. 
Survey, 1906. 
