CORRASION BY GRAVITY STREAMS. 323 



C. Apparent lack of ice corrasion as evidenced by a 

 study of present glaciers.— The channel form is adjusted 

 to the strength of the stream. Upon a reduction of stream 

 volume a readjustment of the channel grade is set up, and 

 the stream appears most inert at the locations of the 

 maximum energy exerted by the larger stream. 



This is the case with present day glaciers. The recent 

 ice floods formed cirques, rock basins, and other forms 

 adjusted to their own size and strength while the deglaciated 

 valleys of to-day evidence the very recent retreat of the 

 last ice flood. The channel grade therefore must now be 

 readjusted. This being so, morainic material must fall out 

 along the old cutting curves, and the ice generally must 

 be most inert at spots like fiord and lake basins. This 

 explains the apparent anomaly otherwise of the association 

 of inert and banded glaciers with cirques and rock basins, and 

 the occupation of old ice cutting curves by moraines. 



Appendix I. — The Path of a Stream Particle. 

 Consider the path of a particle with initial velocity u in 

 a direction at angle 6 to the horizontal and acted on by the 

 constant gravitational force mg, where m = mass, and g 

 the intensity of gravity or force per unit mass. 



Take axes Ox and Oi/, as in Fig. 1, Part I of this Series. 

 The velocity along Ox is unaltered throughout motion as 

 the force is vertical, i.e. the velocity along Ox is u cos 0. 



Therefore the position of the particle after any time t 

 when measured parallel to Ox is given by x = u cos 6t. 



Position of particle after any time t when measured 

 parallel to Oy is given by 



y = u sin 6t.+ \ gt 2 



U— Nov. 3, 1909. 



