BY E. C. ANDREWS. 829 



reversed grades downstream, but unassociated with troughs, are 

 features of flood-channels. 



Alike in the roadside gutter, the incipient canon, the brooklet, 

 the brook, the small and the mighty river, these forms occur over 

 and over again. In each type of channel the dimensions of these 

 peculiar contours are intimately related to those of the accom- 

 panying stream when in flood. Furthermore, such shapes, by 

 direct observation, are known to express the work of the mightiest 

 floods only which obtain in the various localities. And again 

 the situations of such forms are exactly those which might have 

 been easily predicted from considerations of gravitative thrust; 

 that is, they occur : — 



i. At points of marked convergence. 



ii. Along lines working upstream from points of marked 

 convergence. 



iii, Along lines connecting points of marked convergence. 



iv. Above points which, by reason of superior hardness or soft- 

 ness, have allowed of pronounced differential vertical corrasion. 



These " facts of form " suggest that along a channel-floor of 

 definite slope, a flood, or rather a flood-series, descended. The 

 gravitative thrusts of the floods found partial, but not complete, 

 expression in undermining of the channel-sides and in transporta- 

 tion of debris along the base. Relief, however, was in great 

 measure afforded by vertical cutting; and this action ceased only 

 when the vertical component of the gravitative thrust had been 

 expended. A heavy flood-series expressed its maximum vertical 

 thrusts as a succession of basins and troughs along the stream- 

 channels, while subsequent lesser-flood action was incompetent 

 to reduce the grades so formed, and was directed rather to 

 aggradation of channel-base irresrularities. 



Again, forms almost identical with those just enumerated may 

 be seen along shorelines or the canons and valleys but recently 

 vacated by the huge glaciers of the " Ice Age." The grandest 

 contours of fiord cliffs and basins are not only similarly shaped 

 but similarly situated to those formed by ordinary stream-floods. 



