504 NEW ZEALAND SOUND (AND LAKE) BASINS 



grade by aggradation of flood holes. The apprehension of this, 

 fact is absolutely vital to the proper understanding of land forms. 

 One very strong flood will do incalculably more work than a 

 decade of ordinary weather. Thus the author has seen, along 

 an eastern torrent track in New England, a storm which yielded 

 some 10 or 12 inches of rain within an hour. Hundreds of 

 boulders, many tons in weight, were dashed against each other in 

 the creek bed until they had covered one another with small holes 

 from half-an-inch to an inch in depth. Smaller blocks were 

 carried along with such force as to produce loud shrieks as they 

 struck each other. Large basins were ploughed out to such 

 depths along the torrent bed that the later work of the stream 

 has been entirely confined to their aggradation. 



A very powerful stream, therefore, quickly accomplishes its 

 vertical work, and even develops holes below local base-level to 

 depths roughly commensurate with its own height above base- 

 level at that particular spot; a very weak stream, on the other 

 hand, does its vertical cutting much more slowly. After a certain 

 stage of downcutting has been reached, each possesses a grade the 

 lowering of which progresses almost infinitely slowly, and lateral 

 cutting henceforth assumes the ascendancy. This slope, for any 

 stream, at which vertical cutting becomes very inefficient, i.e., 

 along which the heaviest floods alone obtaining over the area can 

 cope with their loads so as to produce any corrasion of their beds, 

 may be called the transitional grade. For weak and strong 

 streams alike all such transitional grades are fairly close approxi- 

 mations to base-level, as will be proven anon; nevertheless they 

 differ widely among themselves. For a very powerful stream, 

 the transitional grade of an insignificant brook would be exces- 

 sively steep, and, if acted upon by such stronger body of water, 

 would be quickly made flatter before lateral corrasion, in its case, 

 gained the upper hand. 



To select a simple illustration : — The normal stream of the 

 giant Amazon River is excessively powerful compared with the 

 mightiest floods of our Eastern New South Wales streams, and 

 would, if turned into their valleys, quickly corrade their short 



