GRAVEL AS A RESISTANT ROCK 495 



that gravel would be one of the most resistant of rocks as far as 

 its relation to the processes of disintegration and removal is 

 concerned. 



Compare, for instance, the relative ease with which weathering 

 and erosion break up and remove the rocks from an area of granite 

 and from one of moderately coarse gravels in a similar situation. 

 In the case of the granite there is always a greater or less number of 

 joints or fissures through which water may enter and perform its 

 work of disintegration either by direct chemical decomposition or 

 by the subsidiary agency of frost. In contrast to this there are 

 the smooth, usually fissureless surfaces of the gravel units. The 

 granite is made up of a variety of minerals, some of which are easily 

 attacked by the weathering agents. Some, it is true, are as resist- 

 ant as the most resistant components of the gravel but in every 

 case these are small, being limited in size by the texture of the 

 granite. There is too, in a rock of complex mineral composition, 

 the factor of pulling apart of the mineral grains by differential 

 expansion and contraction. 



The less resistant minerals, by weathering away and breaking 

 down, leave the harder and more resistant ones free to be removed 

 by the surface waters. Since, in general, the size of the grains is 

 comparatively small, in a granite scarcely exceeding one centimeter 

 in diameter, the resistant materials are readily removed by the 

 streams in the form of sand, while the products of the more thor- 

 ough disintegration of the less resistant minerals are easily carried 

 away in suspension or solution or may even be, in considerable 

 measure, picked up and carried off by the winds. 



Thus we see that a granite is much more vulnerable to the 

 attacks of the weathering agencies than a coarse gravel. What 

 is true of granite is also true in varying measure of any of the less 

 resistant sedimentary or igneous rocks, such as shale, soft sand- 

 stone or limestone, diorite, etc. In the case of quartzite and cer- 

 tain of the lavas it is a question which would disintegrate more 

 rapidly, these or the gravel. The latter has in its favor, as a resist- 

 ant rock, the factor of porosity and the slight effect of insolation. 



All the above considerations apply particularly when the slope 

 is low. On very steep slopes the lack of coherence of the gravel, 



