844 E. B. MATHEAN'S SUBMEKCJED "DEEPS" JJ>! SUS<iUEHA^'^' A KIVER 



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luidei'cutliiig of softer stratn, 

 overlain by more resistant, as at 

 Niagara. 



The petrogJ'apliic featiii'es ai"e 

 luonotoiiously uniform along the 

 ri\ej' banks, giving no suggestion 

 of differences in tlie resistance 

 of adjoining units similar to 

 those found in subsequent water- 

 falls, although the gorge as a 

 whole may be considered, a water 

 gap. The undisturbed graded 

 plain of the valley floor on all 

 sides precludes the assumption 

 of local warping. 



The strike of the schistosity 

 and major jointing are across 

 the course of the river, which in 

 turn shows little or no regard 

 for the minor structures. 



The only suggested condition 

 remaining is that of a '^narrows" 

 produced either by local, as op- 

 posed to stratigrapliic, variations 

 in the liardness of the rocks or 

 by temporary increases in the 

 volume of the river. In such 

 "narrows" it is conceivable that 

 the increased velocity due to local 

 reduction of the cross - section 

 might increase and localize the 

 erosive action of the river. 

 Moreover, the local emponding 

 of the waters upstream may have 

 increased the water gradient 

 enough to give a small but in- 

 creasing downward, rather than 

 lateral, erosion. In this way it 

 is possible to explain the catenary 

 form of the bottom profile, the 



