The large boulders not only act as shelter against rain, but they bind and consolidate by their mere weight the clay upon which they rest.  Hence the materials underlying the boulders become more resistant, and as the surrounding clays are gradually washed away and carried to the streams, these compacted parts persist, and, finally, stand like walls or pillars above the general level.  After a time the great boulders fall off and the underlying clay becomes worn by the rainwash to fantastic spikes and ridges.  In the Val d’Herens the earth pillars are formed

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of the deep moraine stuff which thickly overlies the slopes of the valley.  The wall of pillars runs across the axis of the valley, down the slope of the hill, and crosses the road, so that it has to be tunnelled to permit the passage of traffic.  It is not improbable that some additional influence—­possibly the presence of lime—­has hardened the material forming the pillars, and tended to their preservation.

Denudation has, however, other methods of work than purely mechanical; methods more noiseless and gentle, but not less effective, as the victories of peace ate no less than those of war.

Over the immense tracts of the continents chemical work proceeds relentlessly.  The rock in general, more especially the primary igneous rock, is not stable in presence of the atmosphere and of water.  Some of the minerals, such as certain silicates and carbonates, dissolve relatively fast, others with extreme slowness.  In the process of solution chemical actions are involved; oxidation in presence of the free oxygen of the atmosphere; attack by the feeble acid arising from the solution of carbon dioxide in water; or, again, by the activity of certain acids—­humous acids—­which originate in the decomposition of vegetable remains.  These chemical agents may in some instances, e.g. in the case of carbonates such as limestone or dolomite—­bring practically the whole rock into solution.  In other instances—­e.g. granites, basalts, etc.—­they may remove some of the

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constituent minerals completely or partially, such as felspar, olivine, augite, and leave more resistant substances to be ultimately washed down as fine sand or mud into the river.

It is often difficult or impossible to appraise the relative efficiency of mechanical and chemical denudation in removing the materials from a certain area.  There can be, indeed, little doubt that in mountainous regions the mechanical effects are largely predominant.  The silts of glacial rivers are little different from freshly-powdered rock.  The water which carries them but little different from the pure rain or snow which falls from the sky.  There has not been time for the chemical or solvent actions to take place.  Now while gravitational forces favour sudden shock and violent motions in the hills, the effect of these on solvent and chemical denudation is but small.