SOIL MOISTURE AND TO EVAPORATION. 11 



the rock, exhibited a total moisture content of 5 to 10 per cent of their 

 wet volume. A single sample taken at a depth of 15 cm. contained 

 13.04 per cent of water, and other samples ranged in moisture content 

 from 7 to 12 per cent. It is thus seen that there is considerable 

 variation in contained moisture at the same depth in different places, 

 largely due, no doubt, to the relation of the soil to the surrounding rock 

 fragments and underlying bed-rock or caliche. One digging was made 

 to a depth of 35 cm. and a sample taken from the soil at this depth, 

 lying against solid caliche, which apparently completely closed the 

 opening between the large fragments of volcanic rock which had been 

 followed in the digging, contained 15.16 per cent of water. 



It was intended to make larger excavations and determine moisture 

 conditions at greater depth, but the beginning, on July 15, of the period 

 of heavy rains made this seem of no avail. The structure of the surface 

 layers of the hill, composed, as it is, of mingled rock fragments, offers 

 many chances for water from the surface to find its way to the lower 

 levels along rock surfaces, especially as all the superficial hollows and 

 rock pockets stand full of water for some time after each heavy shower. 

 The soil puddles and becomes itself very slowly permeable to water, 

 but the latter was shown, by diggings made shortly after the first rain, 

 to have attained the depth of the larger rock masses by following down 

 the surfaces of rocks which were exposed above. 



From the moisture determinations which were made it is evident 

 that this soil does contain, during the driest season of the year, rather 

 large amounts of water, and this at no great depth. Spalding (1904) 

 found, about April 24, 1904, that a sample of soil of this same locality, 

 at a depth of 30 cm. , contained 8 per cent of its air-dry weight of water. 

 The same author says, ' 'Another sample from the hill [presumably at 

 the same depth] lost, by heating over an electric stove, 12 per cent of 

 its weight. ' ' From the relations of weight and volume given above it 

 is easy to reduce this result to the approximate percentage by volume 

 under water. As above stated, the per cent of water content calculated 

 on the dry weight of the soil is 17.6 per cent greater than that calculated 

 on wet volume. Thus Spalding's 12 per cent is 1.176 times the corre- 

 sponding water content figured by the method here used, and we have 

 the condition: 1.176v=12, wherein v is the percentage of contained 

 water on the basis of wet volume. From this it appears that v^=10.2 

 per cent. 



It is probable that the method of drying over the electric stove 

 failed to remove all the water from the soil sample, and this may 

 partially account for the fact that the figure just derived is some- 

 what lower than would be expected from the determinations given 



