168 DYNAMICAL GEOLOGY. 



dition of rocks, due to decay through chemical methods, and to the superficial 

 riftings and fractures attending chemical changes, organic growths, freezing, 

 and the alternating of cold with heat occasioned by the sun. 



The methods of demidation are (1) by water-strokes, or the simple impact 

 of water; (2) by abrasion, which includes (a) wear of rocks by means of 

 the stones and earth carried or thrown against rocky surfaces ; and (b) wear 

 of transported stones or grains by their mutual friction or corrasion. By 

 these means much of the shaping of the earth's surface and the trituration 

 of rocks to earth has gone forward. Abrasion becomes a shearing action in 

 planation and terrace-making. 



1. Impact of tvater simply. — In the flow over a smooth surface of rock 

 pure water has no abrading effect. But when thrown in masses, in the form 

 of plunging waves or torrents, into cavities of rocky bluffs or against bold 

 projections, great results may be produced. Blocks of many tons' weight 

 along a shore, if resting on a surface but slightly inclined toward the deeper 

 water, will slip downward with each stroke. 



The force of the impact of flowing water is expressed in pounds, by the general 

 equation P = 0-9702nst?'", in which v is the velocity in feet per second, s is the greatest 

 transverse section of the body in square feet, n a coeflicient varying with the form of the 

 body, the value being ascertained for any particular form by trials ; and 0*9702 is the 

 quotient from dividing the weight of one cubic foot of water (62 J pounds) by 2 ^ (p. 174). 

 Supposing the greatest transverse area to be 1 foot : for a simple plate the value of n is 

 1-86 ; for a cube, 1-46 ; for a sphere, 0-51 ; for some rounded forms, only 0-25. If the 

 hemispherical end of a cylinder faces the current, the impact is half less than if the flat 

 end were in front. In accordance with the above, the force of impact against a flat plate 

 a foot square, in a current of 5 miles an hour (or 7^ feet per second), will be nearly 

 100 pounds ; in one of 20 miles an hour (4 times 5), 16 times that for 5 miles, and so on. 

 On the other hand, if the surface struck is a hemispherical concavity., the impact would 

 be very much greater than for a flat surface, the value of n being about 2 for a hollow 

 hemisphere with the concavity to the current. The principle is illustrated in the connec- 

 tion between form and resistance, or form and velocity, in a boat. 



These results of experiment and mathematical calculation show that while it is not 

 possible to measure the force exerted in the movements of a river, the concavities and 

 deep recesses or channels among the rocks along the sides of a rapid stream afford an 

 opportunity for effective blows. 



2. Abrasion; Corrasion. — The transported sand and gravel which is car- 

 ried by water against the rocks within reach acts like the emery of an emery 

 wheel, yet only under slight pressure. The particles, and especially the 

 pebbles or stones, that are thrown by violent torrents against the surfaces 

 of the solid rock, work more effectively, but less constantly. In a current of 

 given velocity the larger stones carried abrade more rapidly than the smaller. 

 At the same time the transported particles or stones, whether in rivers or 

 on seashores, are wearing one another, and this corrasion tends to reduce the 

 material to that fine impalpable state in which even slow-moving waters will 

 transport them. 



