G. D. Hubbard— Colloids in Geologic Problems. 109 



analysis, all three go straight back to geologic processes, 

 and involve geologic problems. Soils result from the 

 normal geologic decay of rocks, primarily of silicate 

 rocks. In their decay, the elements K. Xa. Ca, and Mg 

 usually go into true solution in their secondary salts, 

 while silica, aluminum, and iron go chiefly into colloidal 

 solution and constitute the ultra-clay material of clays 

 and shales. 



In solutions, the colloidal material can be separated 

 from the crystalline material by the use of the Sharpie's 

 centrifugal machine, capable of producing a force seven- 

 teen thousand times that of gravity, or by the use of the 

 Pasteur-Chamberland filter. This separation can also be 

 brought about by coagulation by the addition of salts, 

 and by dialysis. 



In soils the body is essentially sand and clay, the sand 

 being made of fragments of many kinds of minerals, but 

 mostly of quartz : and the clays mostly of hydrous 

 aluminum silicates, with smaller quantities of aluminum 

 and iron hydroxides. "When the salts mentioned above 

 come to the soils they are carried on through, pro- 

 viding the water has free circulation and drainage 

 below : but if there is insufficient rainfall to equal evapo- 

 ration, then these salts may be left in the soils and be 

 continually carried to the surface by evaporation of the 

 water. The colloidal material, however, will usually be 

 adsorbed and will remain in the soil. Too much of the 

 latter tends to clog a soil and prevent the free and neces- 

 sary movement of air and water. 



This last item becomes particularly troublesome in soils 

 that must be irrigated, for irrigation waters differ from 

 rain waters in carrying both true solution and colloidal 

 solution materials, thus furnishing more material to clog 

 the soil than rain waters. The difficulty is still further 

 increased by the fact that most of the water on irrigated 

 lands is removed by evaporation so that everythii 

 both kinds of solution is left in the soil. The colloids 

 become a nuisance usually much before the salts do when 

 ordinary stream water is used for irrigation. The col- 

 loids tend to cement the soil together some little distance 

 below the surface, usually not beyond the reach of the 

 plow, and produce there a "hard pan" layer. This inter- 

 feres with the -movement of the water, either up or down. 



