232 GROVE KARL GILBERT-DAVIS [Mem '''"Ixxt 



sion was reached by a logical combination of induction and deduction in which both of these 

 unlike processes were duly exercised and neither was allowed to obstruct or to dominate the other. 

 His study of Coon Butte, 10 years earlier, employed a similar combination; but there no assured 

 conclusion was reached, partly perhaps because only a single instance was examined. Would 

 that he had taken occasion to analyze his method in the Alaskan problem, where instances were 

 abundant and where an assured conclusion was attained, as fully as he had done in the Coon 

 Butte problem where, by very reason of the excellence of his method of study, the attainment of 

 a conclusion was impossible. 



A particular instance of the interplay of induction and deduction in the Alaskan problem 

 may be brought forward; it concerns the inequalities in the fiord-trough floors as discovered by 

 sounding. They are explained by following "Gannett's theorem that the glacier-made valley 

 [or fiord trough] is homologous, not with river-made valley, but with the channel made by the 

 river. The bottom of a river channel is not evenly graded like its flood plain, but it abounds in 

 hollows and hills, and the bottom of a glacier channel has irregularities that are similar but on 

 a larger scale" (148). That the more striking of these irregularities may be interpreted as 

 witnesses to the immaturity of the troughs that they interrupt is intimated in later passages. 

 It is worth adding that certain examples of salient irregularities in glaciated trough floors have 

 often been cited by opponents of glacial erosion as preglacial features which, in spite of their 

 small size, glacial erosion could not remove. Such salient are here more reasonably regarded as 

 the comparatively small residuals which survived the great erosion by which the trough as a 

 whole was excavated, and which would have been more completely removed if the great erosion 

 had been carried somewhat farther. 



It is worth recording that Gilbert did not have recourse to down-faulting in explanation of 

 the Alaskan fiords, although that process has been rather frequently called upon in recent years 

 by certain European observers in explanation of Norwegian fiords ; and it is also worth recording 

 that the analysis by which the European observers were led to accept down-faulting was — in its 

 published form at least — by no means so thorough or so critical as the analysis by which Gilbert 

 was led to reject it. He believed that the Alaskan fiords follow the courses of normal valleys of 

 less depth, and that some of the valleys had been eroded along the strike of the strata and others 

 across it; he recognized that in the uplift of the older peneplain to its present plateau altitude, 

 it was possibly "interrupted here and there by faults" (129) ; but he wisely avoids the extreme 

 view that each fiord represents a narrow down-faulted block. This view has had two inter- 

 pretations by European observers : One that the down-faulting was of so recent a date that the 

 glaciers soon followed it and had comparatively little to do with shaping the troughs; the other 

 that the down-faulting was of earlier dat'e than the erosion of the highland peneplain, its effect 

 being to let down narrow slabs of weaker rocks between larger blocks of harder rocks, so that 

 after the mosaic mass was degraded and uplifted, only the excavation of the weaker slabs by a 

 combination of normal and glacial erosion was needed to produce the fiords. Neither of these 

 interpretations was mentioned in Gilbert's analysis; but his phrase regarding the interruption 

 of the uplifted peneplain only "here and there by faults" suffices to show that neither would 

 have been acceptable. 



PHYSIOGRAPHIC ITEMS 



On the other hand, Gilbert's statement as to the origin of the stream courses by which the 

 uplif ted peneplain was eroded in preglacial time contains a peculiar item : 



The master streams were largely consequent to the seaward slope of the old plain, but were in part directed 

 to lines of strike; and minor streams were adjusted to rock structures (130). 



If taken literally, the first clause implies that the peneplain, or "old plain" as it is here 

 called, was so smooth and its slanting uplift was so rapid that the master streams which had 

 drained it during its degradation were as a rule extinguished and replaced by new streams 

 "consequent to the slope." As nothing is directly known about the master stream courses on 

 the peneplain, and as the fiords to-day exhibit a considerable irregularity in their directions, it 

 seems permissible at least to suppose that the region at the time of its most advanced degreda- 

 tion was not a plain but a peneplain, for some of its inequalities were recognized to be so large 



