PHYSICAL COMPOSITION OF SOILS. IO i 



and 7 % more than the latter. But No. 246 is a highly ferruginous 

 clay, in which the ferric hydrate is in a very finely divided condition, 

 and materially influences the physical qualities of the clay substance. 

 Were it all accumulated in the " clay," it would diminish the percen- 

 tage of true clay by 11.75%, reducing tne clay-percentage 1028.5% 

 which accords more nearly with the soil's only moderate adhesiveness, 

 and not excessively heavy tillage. 



But it must be remembered that the iron oxid shown in the 

 analysis is not nearly always in this finely diffused condition. 

 Frequently it incrusts the sand grains ; quite commonly it forms 

 small concretions of limonite, which themselves act as sand 

 grains; and again, it may be present in the form of ''black 

 sand " or magnetic oxid, as is commonly the case in California 

 and on the Pacific slope generally. To take this point properly 

 into account, therefore, it would be necessary to determine the 

 amount of ferric hydrate actually present in the " clay " as 

 separated by subsidence of the granular constituents. 



Other substances. This circumstance as well as the inevi- 

 table presence of other modifying substances, clearly shows the 

 desirability of being enabled to examine the physical properties 

 of this " clay '' directly, by collecting its entire amount as ob- 

 tained in analysis, instead of merely determining it by weighing 

 fractional portions. When this is done the analysis is much 

 more valuable as indicating the true tilling qualities of the 

 land. The increase of bulk suffered by this substance after 

 wetting, is a very fair index of its content of true clay, and is 

 preferable to the chemical analysis proposed by some investi- 

 gators. For it is quite impossible to distinguish the silica and 

 alumina derived from the kaolinitic substance proper, from that 

 which is due to the decomposition of zeolites. 



It is possible, however, to determine the possible maximum 

 of the kaolinite ingredient by taking into consideration the 

 quantitative ratio according to which silica and alumina com- 

 bine to form it, viz., approximately 46 /f of the former to 40 of 

 the latter, the rest being water. By using this calculation we 

 can often demonstrate clearly the presence in the " clay " of 

 considerable amounts (up to 33^ ) of aluminic hydrate: since 

 no zeolitic mass can contain as much alumina as does kaolinite. 

 Whether the aluminic hydrate be in the form of gibbsite. 



