CORRASION BY GRAVITY STREAMS. 263 



2. Gravity acts freely as opportunity offers and aims at 

 producing a cut normal to the earth's surface. With no 

 obstacle resisting its pressure it causes a body to fall freely. 

 Upon a declivity it expresses its power partly as flow and 

 partly as free falling, the latter factor rising as the slope 

 increases, for gravity always attempts an approximation 

 to its ideal, viz.: — vertical motion. 



3. In a stream, pressure as weight begets increased 

 freedom among the lower textural units of the mass. A 

 get-away is present ; flow is thus set up and the surface 

 slope in a downstream direction must be preserved. Where 

 a channel narrows in, the relatively small stream cross- 

 section here must be associated with increase of velocity. 

 This is so both from a consideration of the volumes and a con- 

 sideration of the constancy of the time factor involved. 

 This is one of the marked features of a stream. 



4. The channel "Broad" is fed from an upstream mass 

 of less cross-section. To counterbalance this enforced 

 increase of volume locally, the velocity must here be pro- 

 portionately reduced as compared with that of the upstream 

 feeding mass. 



5. If friction were reduced to zero, it is apparent as 

 W.D.Johnson (pp. 576-577) suggests, 1 that the fastest 

 motion would be vertically beneath the centre and at the 

 base. But in nature friction is pronounced at the base of 

 streams, therefore the maximum rate of flow is at some 

 distance above the base and away from the sides. 



6. This is also a characteristic of streams. They must 

 follow the lines of least resistance. At such a point the 

 stream is influenced in a double way. Pressure from above 

 and from the sides induces freer motion among the textural 

 units of the glacier, the freedom varying with the pressure, 

 and the tendency also is to fall freely under the action of 



1 See also Andrews (a) pp. 37, 38. 



