THE SOIL. — ITS FORMATION AND COMPOSITION. 79 



aiid its gases destroy the smooth surface of the rock and pro- 

 duce hollows, then the seeds of lichens and mosses which 

 are floating about in the air, adhere to it, grow and in- 

 crease, appropriating its materials and then die, giving it a 

 covering of organic matter.* By their decay also, carbonic 

 acid is generated, which contributes to accelerate the decom- 

 position of the rock, and thus a soil is gradually produced. 



113. In countries like Egypt, in which the air is dry and 

 rain is almost unknown, decay does not proceed with such rapid 

 steps as under our moist and changeable atmosphere. The 

 surifaces of monuments in Egypt remain scarcely touched by 

 the passing wings of time, while with us, all the massive 

 buildings erected by the piety or ambition of our forefathers 

 have been swept over as by a whirlwind. 



114. The rapidity with which rocks wear and crumble 

 depends in a great degree upon their composition ; while the 

 cinders of lava slowly fall into powder, the decay of granite 

 and clay-slate proceeds more rapidly, and sandstone and basalt 

 are still more quickly reduced to dust.* Darwin, in his in- 

 teresting nan-ative of the voyages of the "Adventm'e and 

 Beagle," gives a sketch of the vegetation of some of the 

 coral islands in the Pacific Ocean, which shows the manner 

 in which these singular- formations receive their covering of 

 plants. '* The cocoa-nut tree," he says, " at the first glance, 

 seems to compose the whole wood; there are however six 

 other kinds, one of these grows to a very large size, but 

 from the extreme softness of its wood is useless ; another sort 

 aflfords excellent timber for ship-building. Besides the trees 

 the number of plants is exceedingly limited, and consists 

 of insignificant weeds. In my collection, which includes, I 

 believe, nearly the perfect Flora, there are twenty species 

 without reckoning a moss, lichen, and fungus. To this number 

 two trees must be added, one of which was not in flower, 

 and the other I only heard of. The other is a solitarj^ tree 

 of its kind in the whole group, and grows near the beach 

 where without doubt the one seed was thrown up by the 



• I have closely watched the progress of decay in some of our clay- 

 slate rocks, and the powerful effects in their destruction produced by 

 the influence of plants. One of those which I have most commonly 

 observed is the wild sorrel, Rumex acetosella^ the roots of which I have 

 found forcing themselves for a great lengtli beween the layers of the 

 rock, and in their progress loosening and finally tearing them asunder. 

 The inferior orders of animals also assist in the work. 



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