DL'TTON. 



ANALYSIS OF CORRASION. 157 



CORRASION. 



Mr. G. K. Gilbert has embodied in bis admirable monograph on the 

 Henry Mountains, a chapter on Laud Sculpture, wbicli sets forth in 

 most logical and condensed form the mechanical principles which enter 

 into the problems of erosion. In bis analysis may be found a discussion 

 of the conditions under which the sculpturing forces and processes 

 achieve such peculiar results as we observe in tbe Plateau country. 



The perusal of that chapter "will give to the geologist's comprehension of 

 tbe subject a most delightful definiteness and precision, and the reader, 

 however learned he may be, will take great satisfaction in finding a 

 subject so complex made so intelligible. The principles laid down by 

 Mr. Gilbert will be adopted here and applied. For that purpose I quote 

 from the chapter referred to such statements as are of immediate service. 



The mechanical wear of streams is performed by the aid of hard mineral fragments 

 carried along by the current. The effective force is that of the current ; the tools are 

 mud, sand, and bowlders. The most important of them is sand; it is chiefly by the 

 impact and friction of grains of sand that the rocky beds of streams are disintegrated. 



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 and load is consumed 

 in the translation of the water aud load, and there is none applied to corrasion. If 

 it has an excess of load, its velocity is thereby diminished so as to lessen its competence 

 and a portion is dropped. If it has less than a full load, it is in condition to receive 

 more, aud it corrades its bottom. A fully-loaded stream is on the verge between cor- 

 rasion and deposition. * * * The work of transportation may thus monopolize ;•, 

 Stream to the exclusion of corrasion, or the two works may be carried forward at the 

 same time. 



The rapidity of mechanical corrasion depends on the hardness, size, and number 

 of the transient fragments, on the hardness of the rock-bed, and on the velocity of the 

 stream. * * * The element of velocity is of double importance, since it determines 

 not only the speed, but, to a great extent, the size of the pestles which grind the rocks. 

 The co-efficients upon which it [velocity] in turn depends, namely, declivity and 

 quantity of water, have the same importance in corrasion that they have in transpor- 

 tation. 



Let us suppose that a stream endowed with a constant volume of water is atsomo 

 point continuously supplied with as great a load as it is capable of carrying For so 

 great a distance as its velocity remains the same, it will neither corrade nor deposit, 

 but will leave the declivity of its bed unchanged. But if in its progress it reaches a 

 place where a less declivity of bed gives a diminished velocity, its capacity for trans- 

 portation will become less than the load, and a part of the load will be deposited. Or 

 if in its progress it reaches a place where a greater declivity of bed gives an increased 

 velocity, the capacity for transportation will become greater than the load and there 

 will be corrasion of the bed. In this way a stream which has a supply of debris equal 

 to its capacity tends to build up the gentler slopes of its bed and to cut away the 

 steeper. It tends to establish a single uniform grade. 



Let us now suppose that the stream, after having obliterated all of the inequalities 

 of t he grade of its bed, loses nearly the whole of its load. Its velocity is at once accel- 

 erated and vertical corrasion begins through its whole length. Since the stream has 

 the same declivity, and consequently tho same velocity, at all points, its capacity for 

 corrasion isevery where the same. Its rate of corrasion, however, will depend upon the 

 character of its bed. Where the rock is hard, corrasion will be less rapid than where 



