BY E C. ANDREWS. 503 



transitional grade known for anyone member : this by use of the 

 formula — (Transportation) varies as (Velocity) 6 . 



Stream grades are determined by the mightiest floods only, the 

 normal stream being mainly functional in smoothing over the 

 irregularities marking the approximation of floods to base-level. 

 Exti'eme types of these activities are to be found in ordinary 

 roadside flood-streams, the Amazon River in flood, the ice-floods 

 of the recent great Glacial Period, and the storm sea-wave. 



Thus several distinct plateaus excavated in a geological complex, 

 and arranged in vertical succession, without faulting, represent as 

 many cycles of erosion forming plains at or near base-level, later 

 elevations giving the present heights to the upland levels. 



Flood and drought-stream grades. 



Let us consider the action of streams on a raised plain. Such 

 a plain may be, structurally, homogeneous, a series of horizontal 

 strata varying in texture, or a geologic complex. 



(a) Action of streams on a homogeneous mass upon uplift. 



The quickening of stream velocity by the uplift allows of the 

 early incising of the raised surface. Every stream seeks to 

 maintain a grade along which to accomplish efficient work, yet, 

 at the same time to approximate as closely as possible to base- 

 level. The establishment of a grade along which the stream can 

 only just cope with its load leads to main stream action being 

 directed towards lateral cutting. These truths follow immediately 

 from gravitative considerations. The stronger the stream, the 

 flatter the grade along which appreciable work, as corrasion, can 

 be accomplished. Mighty floods are enabled to excavate deep 

 holes below local and even main base-level, thereby demonstrating 

 their ability to accomplish important work along much flattened 

 grades. The depths of these holes excavated below the temporary or 

 main base-levels are measures of the strength — i.e., volume (depth) 

 and velocity of the flood. This leads to the important considera- 

 tion that present channel grades are not meastires of the strength 

 of normal stream action, but that of the mightiest floods. Normal 

 stream activities are mainly functional in restoring a uniform 



