THE GRAVEL-SLIDE COMMUNITY. 



89 



effectiveness of this layer is shown in the following experiment. Two 

 metal cylinders, 8 inches high and 5.5 inches in diameter, closed at one 

 end, were filled with wet soil and one of them was covered with a layer 

 of these coarse gravel particles to a depth of an inch. After an equal 

 exposure to evaporation for a period of 30 hours, it was found that 

 the one container had lost 202 grams of water, which was more than 8 

 times as much (24 grams) as evaporated from the soil covered with 

 the gravel mulch. In nature, finer particles occupy the interstices be- 

 tween larger ones, and hence the gravel mulch must be much more 

 efficient. 



Below this surface gravel are about 4 inches or more of fairly well 

 decomposed rock, a mixture of coarse gravel and sand. On older 



Table 18. — Water-content of the soil of the gravel-slide and half-gravel-slide during 1918. 



slides this layer extends much deeper. Below this the soil changes 

 from a dark brown to a more reddish color and consists of fairly well 

 decomposed granite, which becomes less broken up as one goes deeper. 

 At 2 to 4 feet in depth it changes into almost soHd rock. The roots 

 show a marked tendency to follow the cleavage planes of the rock. 

 Below 4 to 6 inches the soil is remarkably compact, and it is necessary 

 to remove it with a pick, this sometimes being accomplished with con- 

 siderable difficulty. 



An examination of the weekly soil-moisture determinations in table 18 

 shows that while the water-content is at no time high, it is rather uni- 

 formly distributed throughout the first 18 inches of soil. Owing to the 

 extreme irregularity in degree of fragmentation of the rock particles 

 and to the heterogeneous nature of these soils, moisture-equivalent 

 determinations are not given. The amount of non-available water 

 in the gravel-slide soils was found to vary from 1.9 to 7.8 per cent; 

 in the half-gravel-slide from 2.0 to 8.6 per cent; and in the soils of the 

 forest floor from 2.7 to 10.5 per cent (page 109). 



The rather high evaporating power of the air as measured by non- 

 absorbing atmometers is given in table 19. 



