CHAPTER XIII. 85 



your shoes. The same thing may ue observed by running water on a 

 grass plat. There will never be any mud. It does not matter how 

 much water is put on. The only effect is to make the soil more re- 

 ceptive and to render the sod soft and elastic. The same water on 

 bare soil, hard or cultivated, will make mud. 



These experiences can be easily demonstrated, even within the 

 limits of a city. They are all germane to forestry. 



VALUE OP HUMUS. 



No one observing these results can overlook the importance of hu- 

 mus in the soil. The forest makes and protects the humus. When the 

 forest is burned so is much of the humus, and sometimes all of it. A 

 burned over forest, whether the trees are destroyed or not, cannot be 

 expected to take care of as much rainfall as an unburned one. 



Torrent action may and does occur from fire, without the destruc- 

 tion of all the trees. The destruction of the humus alone will account 

 for a large increase in storm off-flow, and a necessary diminution of 

 the springs. There are a number of instances in France of torrents 

 created by forest destruction and of their subsequent extinction by a 

 renewed forest growth. I have seen quite a number of the charcteristic 

 torrent cones at the outlet of mountain canyons to plains not now 

 active, and the stream that made them again perennial. 



TORRENT CONES. 



The torrent cones have a very important influence upon the water 

 supply in Southern California. These cones are composed mainly of 

 boulders at the outlet of the canyons and the change of grade from the 

 mountain to the valley. As you go out into the valley, the boulders 

 become smaller and more and more mixed with gravel and sand. At 

 the lower end of the torrent cones there is all sand. Far out at the 

 end of our storm flows, we find silt and clay. Tremendous deposits of 

 clay are found at present and former river outlets to the sea along 

 the county coast. 



The torrents grind the boulders into gravel, and the gravel into 

 sand, dropping the load both with the diminished grade and the dimin- 

 ished water flow. The water flow diminishes with every foot it runs 

 on the torrent cones and washes. Our storm off-flow is well-known to 

 be entirely of mountain origin. It is not so well known that our entire 

 summer supply is the rainfall of the mountains. 



