ioo THE BOOK OF NATURE STUDY 



Gravitation is the force which is most active in transport, 

 but it may act through various agents, or it may be operative by 

 itself alone. For instance, on a mountain side fragments broken 

 off to make screes are always in unstable equilibrium, and the 

 least impulse sets them travelling down hill. Even if there is no 

 other cause, the loading of the upper part by fresh frost-falls is 

 sufficient, and it is well known that roads crossing screes are very 

 liable to be blocked in frosty or wet weather. Indeed, there is one 

 case known to the writer where, when the road needs mending, a 

 hole is made in the boundary wall and the scree allowed to flow 

 out on the road. The metal is just spread, the wall mended, and 

 the deed is done. In the normal course the material travels 

 down until it reaches a stream, which then undertakes the sub- 

 sequent transport. 



But in many types of country it will only be possible to start 

 with the material found in a stream, to observe it being carried 

 downward, and to reason back to its derivation from the place 

 where the parent rock is exposed. From a stream in spate it will 

 be possible to collect muddy water and thus to test the power of 

 running water to carry mud and sand in suspension. The trans- 

 port of larger fragments by rolling along the bottom is less easily 

 demonstrated, though it can often be heard; still, the fact that 

 such fragments are generally present in a stream bed, and that 

 they are usually rounded and rolled, is quite obvious and may be 

 reasoned upon. It may thus be inferred that the stream utilises 

 the products of disintegration, and, knocking off their angles and 

 edges as they are rolled along, it impresses upon them its own 

 " tool-mark/' the rounding and rolling of the fragments, until 

 they pass into pebbles and gravel. 



It will also be possible to show the relation in form and size 

 of pebbles to the hardness of the rock of which they are made, 

 and the control exercised on their form by the shape of the 

 original fragments, a consequence of the rocks from which they 

 are made. At the same time, the share that transported material 

 must take in deepening the water course may be pointed out, 

 and it may be possible to show the accelerated rate of work at 

 certain parts of the stream, such as rapids or waterfalls, where the 

 velocity is specially great (Fig. 69), or its special type where eddies 



