Nor is good drainage favourable to chemical actions, for water is the primary factor in every case.  Water takes up and removes soluble combinations of molecules, and penetrates beneath residual insoluble substances.  It carries the oxygen and acids downwards through the soils, and finally conveys the results of its own work to the rivers and streams.  The lower mean temperature of the mountains as well as the perfect drainage diminishes chemical activities.

Hence we conclude that the heights are not generally favourable to the purely solvent and chemical actions.  It is on the lower-lying land that soils tend to accumulate,

35

and in these the chief solvent and the chief chemical denudation of the Earth are effected.

The solvent and chemical effects which go on in the finely-divided materials of the soils may be observed in the laboratory.  They proceed faster than would be anticipated.  The observation is made by passing a measured quantity of water backwards and forwards for some months through a tube containing a few grammes of powdered rock.  Finally the water is analysed, and in this manner the amount of dissolved matter it has taken up is estimated.  The rock powder is examined under the microscope in order to determine the size of the grains, and so to calculate the total surface exposed to the action of the water.  We must be careful in such experiments to permit free oxidation by the atmosphere.  Results obtained in this way of course take no account of the chemical effects of organic acids such as exist in the soils.  The quantities obtained in the laboratory will, therefore, be deficient as compared with the natural results.

In this manner it has been found that fresh basalt exposed to continually moving water will lose about 0.20 gramme per square metre of surface per year.  The mineral orthoclase, which enters largely into the constitution of many granites, was found to lose under the same conditions 0.025 gramme.  A glassy lava (obsidian) rich in silica and in the chemical constituents of an average granite, was more resistant still; losing but 0.013 gramme per square metre per year.  Hornblende, a mineral

36

abundant in many rocks, lost 0.075 gramme.  The mean of the results showed that 0.08 gramme was washed in a year from each square metre.  Such results give us some indication of the rate at which the work of solution goes on in the finely divided soils.[1]

It might be urged that, as the mechanical break up of rocks, and the production in this way of large surfaces, must be at the basis of solvent and chemical denudation, these latter activities should be predominant in the mountains.  The answer to this is that the soils rarely owe their existence to mechanical actions.  The alluvium of the valleys constitutes only narrow margins to the rivers; the finer debris from the mountains is rapidly brought into the ocean.  The soils which cover the greater part of continental areas have had a very different origin.