E. W. Hilgard — Peculiarities of Rook - Weathering. 265 



since a root cannot perform its functions in the absence of the 

 oxygen of the air, it is clear that the air readily penetrates to 

 the depth shown ; and in many cases observed, even to as much 

 as twenty feet and more. And since both air, moisture and 

 humus are found at these depths, this implies not only that 

 the farmer in the arid region can fearlessly plow to any practi- 

 cable depth, but also that the mass of his available soil is from 

 three to five times as great as that of an equal area of land in 

 the humid East. In other words, he has several farms one 

 above another, instead of a single one with one or two feet 

 of available soil and subsoil. 



If this be so, then it might be allowable in the arid region 

 to use the regolith materials thrown up from cellars directly as 

 cultivable soils. Daily experience shows this to be the case. 

 Except where heavy clay soils prevail, or where wet macera- 

 tion has at some time consolidated the subsoil mass, the mate- 

 rials from eight or ten feet depth dug out of cellars or the 

 foundations of houses can oftentimes be directly used as 

 surface soils in garden and fields ; and the farmer in grading 

 his land for irrigation ordinarily excavates it as deeply as may 

 be necessary without fear of the " raw subsoil," as would be 

 needful in the humid East. The prompt natural afforesting 

 of the placer and even hydraulic mines in California teaches 

 the same lesson. 



Searching for the physical and chemical causes of this state 

 of things, we see at once that it is mainly referrible to the per- 

 vious, pulverulent nature of the regolith, which is itself the 

 result of the absence or great deficiency of plastic clay. It is 

 immaterial whether or not this is concurrent with a slow rate 

 of kaolinization, as the microscope seems to indicate. It is 

 quite certain that the plastic clay substance, the "colloidal 

 clay," is formed only in small amounts ; and this is quite 

 consonant with w T hat we know of the transformation of kaolin 

 into plastic clay. Mere pulverization will not accomplish this; 

 under wet trituration* it is certainly at least a physical 

 hydration process, which does not occur in the case of other 

 fine powders. It is true that the latter may be molded into 

 shape when wetted ; but that shape is not retained after drying 

 if any pressure is applied, while clay proper dries into a hard, 

 resistant mass, such as in arid climates like that of Mesopo- 

 tamia has not even required burning to retain the Assyrian 

 inscriptions for thousands of years. These ancient nations 

 understood the peculiar character of plastic clay better than 

 those who to-day contend that its plasticity is simply due to 

 the fineness of its particles ; for we nowhere find that chalk or 



* See Johnson and Blake, this Journal, May, 1867. 



