195 



position for twenty-four hours, after the expiration of which time the 

 water, together with the clay still in suspension, is syphoned off. It may 

 be necessairy to refill the cylinder with water and repeat several times the 

 process of shaking, subsidence, and drawing off, until no more clay can be 

 removed. All the liquid thus drawn off is evaporated, and the clay which it 

 retained in suspension is then weighed. The sediment in the cylinder is 

 again treated with water and shaken as before, but now the liquid is 

 syphoned off after a subsidence of 100 seconds, and this operation is re- 

 peated until a practically clear liquid comes over. What now remains in 

 the cylinder is washed out, dried, weighed, and returned as " very fine 

 sand," while that which passes over with the syphon-water is again re- 

 turned to the cylinder for a fresh treatment as before, except that a 1,000 

 seconds subsidence is now permitted. What remains in the cylinder at 

 this stage is called " silt," and that which is syphoned off is " fine silt," 

 both being now dried and weighed. 



The following scheme illustrates the entire sifting and sedimentation 

 process : 



Wet sifting. Wet sifting and sedimentation. 



Field 

 sample 



Dry sifting. 



pebbles 

 > 3 mm. 



true soil 

 < 3 mm. 



f gravel 



3 1 mm. 



f coarse gravel 

 3 2 mm. 



fine gravel 

 2 1 mm. 



coarse sand 

 1 mm. 



f medium sand 

 5 '25 mm. 



earth J fine sand 



< 1 mm. | -25 '1 mm. 



I 



very fine sand 

 *1 '05 mm. 



silt 

 05 -Olmm. 



fine silt 



01 -005mm. 



clay 

 _< -005 mm. 



EESULTS OF MECHANICAL ANALYSES. 



fine earth 

 < mm. 



By wet sifting. 



100" sedimenta- 

 tion. 



1000" 



24 h. 



in suspension 



In most of the samples collected, and enumerated in the foregoing 

 pages, where any mechanical separation at all has been made, it has been 

 only of a rudimentary description; that is to say, it has consisted simply 

 in grading the soil into two portions, the one comprising the " fine earth " 

 and the other the coarser soil. And yet, elementary though these opera- 

 tions have been, they have elicited figures that do not entirely lack 

 interest. For instance, if we look at the soils taken from the south- 

 western corner of the Colony, where the Malmesbury and Table Mountain 

 geological series prevail, we find, on the whole, a coarser type of soil than 

 that of the area within which, for the most part, the Bokkeveld series 

 occurs; and this again, yields a soil of coarser texture than the rest of 

 the country from which soils have been examined, and covered chiefly by 

 rocks of the Karroo system, more particularly by those of the Beau- 

 fort series. 



