REPORT OF THE CHIEF ASTRONOMER 627 



SESSIONAL PAPER No. 25a 



crosses the Coast range of British Columbia, are considered by Smith as ante- 

 cedent to the late Pliocene uplift of the Cascade-Coast chain of mountains.* 



The foregoing, rather liberal quotations from Russell, Willis, and Smith, 

 show that these authors are in substantial agreement so far as the essentials 

 of the later geological history of the Cascades are concerned, except that Russell 

 refers much of the work of developing the Cascade peneplain lowland to the 

 Miocene, while the other two authors consider that a truly mountainous topo- 

 graphy characterized the region at the opening of the Pliocene. Besides its 

 interest for the physiographer this skilfully presented hypothesis has great 

 importance, if true, for the dynamic and historical geologist. This is not the 

 place for its full discussion hut the present writer is impelled to sketch the 

 grounds for quite thorough-going disagreement with this hypothesis of the Cas- 

 cade mountain system. 



In the first place, the evidences for the existence of any general peneplain 

 over the Cascades at any time in the history of the system seem extremely 

 weak. Mere truncation of folds has no positive importance in the problem, 

 for mountain-folds are effectually truncated when maturity is reached in a 

 first erosion-cycle. The mere accordance of summit levels among the peaks is 

 likewise to be expected at maturity of dissection in any range of alpine com- 

 plexity, as will be shown in a succeeding section of this chapter. 



Rather extended stream adjustment to soft rock-belts must be expected in 

 a mountainous region which has reached maturity in a second cycle. This 

 well known criterion is scarcely more than mentioned in any of the three 

 papers quoted. The present writer believes, as a result of nearly three seasons' 

 study in and near the Cascade field, that such evidence as there is on this 

 point is against the idea of peneplanation in any part of Tertiary time. 



Most stress is laid by Willis and Smith on the occurrence of flat-topped 

 areas of relatively large size within the distinctly folded zone of the Miocene 

 basalts. f There is no question that the anticlines of this district are truncated, 

 but there is no question that it is dangerous to extrapolate on the curve of the 

 profile in this small district out westward across the great Cascade range, in 

 which similar points d'appui for this bold hypothesis of Pliocene peneplanation 

 practically fail — fail according to Willis's own statement in the first of the 

 foregoing quotations. There is no certainty that the local peneplain represented 

 on the truncated arches of the basaltic territory was not formed far above 

 sealevel or such a general baselevel as we must ascribe to the Cascade region 

 during Pliocene peneplanation of the whole range. The recent studies of 

 Passarge and Davis seem to prove the possibility of ' leveling without base- 

 leveling ' over large tracts of arid mountain-land. There is reason to think 

 that the belt east of the present high Cascades may have been dry and subject 

 to heavy wind-erosion for a comparatively long time. Under the control of the 

 wind in an arid or subarid district newly uplifted rock-folds would suffer 



* G. O. Smith and B. Willis, op. cit., and G. 0. Smith, Bull. 235, U.S. Geol. Survey, 

 1904, p. 90. 



fProf. Paper No. 19, U.S. Geol. Survey, 1903, p. 26; cf. first quotation from Willis. 



-!. r >a — vol. iii — 41 



