Forest physiography; physiography of the United States and principles of soils in relation to forestry . n water the different grain sizes willsettle according to their weights. A successful outcome requires theremoval of the clay by repeated sedimentations of the non-clay material,the decantation of the water in which the suspended clay particles areheld, and the final sedimentation of the coarser grades. The necessityfor the removal of the clay is due to the greater viscosity of the waterin which the clay is suspended and its interference with normal andaccurate sedimentation. The clay itsel
Forest physiography; physiography of the United States and principles of soils in relation to forestry . n water the different grain sizes willsettle according to their weights. A successful outcome requires theremoval of the clay by repeated sedimentations of the non-clay material,the decantation of the water in which the suspended clay particles areheld, and the final sedimentation of the coarser grades. The necessityfor the removal of the clay is due to the greater viscosity of the waterin which the clay is suspended and its interference with normal andaccurate sedimentation. The clay itself is determined from the severalclay waters by evaporation of the water. Precipitation will not suffice, forthe finest colloidal clay will not subside for years, — a condition thoughtto be due to a change in its physical and possibly in its chemical nature.^A defect of the subsidence method is the impossibility of abstracting allthe clay, a defect the more serious because of the high importance of the 1 W. H. Brewer, On the Suspension and Sedimentation of Clays, Am. Jour. Sci., vol. 2g,1885, p. Fig. 12. — Elutriator (Hilgards) in position for soil analysis. I04 FOREST PHYSIOGRAPHY clay constituent of soils (p. 35). Some clay particles are invariablycarried down by heavier constituents and deposited with them. A similarresult does not occur in elutriation because of (i) the agitation (whichprevents flocculation into heavy aggregates) of the ascending currentand (2) the grain sizes are expelled in reverse order, the finest first, andso on. Purpose of a Soil Analysis The physical analysis of a soil is not alone for the purpose of findinga name for it in the series proposed in the table, Appendix A. This isthe least of its purposes. It aims, in addition, clearly to present thecontrolling constituent among the soil grains. Soils are not generallycomposed of grains rather equally distributed among the several particular size usually predominates, and gives the soil
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