The Birth-Time of the World and Other Scientific Essays. John Joly

slopes. Cliffs stand shattered and ready to fall in ruins. And

       here the forces at work readily reveal themselves. An occasional

       wreath of white smoke among

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      the far-off peaks, followed by a rumbling reverberation, marks

       the fall of an avalanche. Water everywhere trickles through the

       shaly _débris_ scattered around. In the full sunshine the rocks are

       almost too hot to bear touching. A few hours later the cold is

       deadly, and all becomes a frozen silence. In such scenes of

       desolation and destruction, detrital sediments are actively being

       generated. As we descend into the valley we hear the deep voice

       of the torrents which are continually hurrying the disintegrated

       rocks to the ocean.

      A remarkable demonstration of the activity of mechanical

       denudation is shown by the phenomenon of "earth pillars." The

       photograph (Pl. IV.) of the earth pillars of the Val d'Hérens

       (Switzerland) shows the peculiar appearance these objects

       present. They arise under conditions where large stones or

       boulders are scattered in a deep deposit of clay, and where much

       of the denudation is due to water scour. 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'Hérens

       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