242 RECOKD OF SCIENCE FOR 1887 AND 1888. 



ican geologists than Irom those of any other country, and the greater 

 part of our knowledge of the subject must be credited to American in- 

 vestigators. Thus, it was American geologists employed in the caiion- 

 cut plateaus and mountains of the western Territories who discovered 

 that the water of a river is not so much the agert of corrasion (which 

 is one of the principal modes of degradation) as the vehicle by which 

 the agent is rendered operative, and that the real agent of corrasion 

 is the sand or other material with which the water of rivers is loaded. 

 Pure water is i)ractically impotent as a corrading agent; but when fur- 

 nished with tools in the form of sand grains it rapidly cuts away the 

 hardest rocks. And this is only one of many American contributions 

 to the subject. 



Until recently it has been commonly believed that while the sand 

 grains and other matter held in suspension in river water directly in- 

 creases corrasion, there is an indirect diminution in corrading ca])acity 

 going with the addition of such matter, due to the absorption of a por- 

 tion of the energy of the stream in transporting the solid matter. Ac- 

 cording to one of the highest authorities on the subject, " where a 

 stream has all the load of a given degree of comminution which it is 

 capable of carrying, the entire energy of the descending water an<l load 

 is consumed in the translation of the Avater and load, and there is none 

 applied to corrasion. - - - The work of transportation may thus 

 monopolize a stream to the exclusion of corrasion, or the two works 

 may be carried forward at the same time." * The same geologist has as- 

 sumed the practical equality of the two elements, and thus that corra- 

 sion is little affected by load. Such has been the state of opinion on 

 this subject for a decade. 



Within a few mouths another American contribution of the first iiD- 

 portance has been made to this subject : In a paper read before the 

 National Academy of Sciences in November, 188S, Powell i)ointed out 

 that there is a distinction between the sediment rolled or pushed along 

 the bottom of a stream, and that which is of such degree of comminu- 

 tion as to float in the running water; and that to the extent that ma- 

 terials are rolled along the bottom by impact the energy of the water 

 is indeed utilized in transportation, as held by Gilbert and others, but 

 that to the extent that transportation is accomplished by flotation, tne 

 gravity of the particles themselves is the entire force of transportation ; 

 so that w^hateveris driven is transported by the energy of the water, while 

 whatever floats is transported by its own inherent gravity. Thuswithiu 

 certain wide limits, and under certain conditions of comminution of the 

 load, the energy of a stream would appear not to be diminished but on 

 the contrary greatly increased by the addition of load. If the law thus 

 formulated be valid (and its validity has not thus far been questioned) 

 its discovery will become of great and wide-reaching practical value to 



* Gilbert, Geology of the Heury Mouutains, 1877, p. HI. 



