DESTRUCTIVE EFFECTS 1 23 



is increased by the acids of vegetable decomposition, which it 

 absorbs in its passage through the soil. Recent observations 

 show that the decay of animals in the deep sea is an agent of no 

 mean importance in promoting the chemical changes which there 

 take place. But even living animals and plants play a part in 

 the work of disintegrating rocks that should not be overlooked. 

 Seeds germinating in the crevices of rocks, or the roots of trees 

 which invade such crevices from above, wedge the rocks apart 

 with the same irresistible power as is displayed by frost, and often 

 large areas of rock are thus most effectively broken up. The 

 roots of living plants also secrete an acid, which dissolves out 

 some of the soluble constituents of rock, thus adding a chemical 

 activity to the wedge-like mechanical effects of growth. 



Many marine animals bore into rocks, even the hardest, and 

 cause them to crumble, and on the land great numbers of animals 

 continually bore and tunnel through the soil, allowing a freer 

 access of air and water. In the tropics the soil is fairly alive with 

 the multitude of burrowers. Earthworms are among the most 

 important agents in work of this kind, and the last of Mr. Dar- 

 win's books was a most interesting one upon the geological work 

 of worms. The worms swallow quantities of earth, for the sake 

 of the organic matter which it contains, and grind it exceedingly 

 fine in their muscular gizzards. This ground-up soil is always 

 deposited on the surface, in the form of the coiled " worm-cast- 

 ings," so abundant in grassy places. Worms are thus continually 

 undermining the soil, bringing up material from below and depos- 

 iting it on the surface, while, by the collapse of the old burrows, 

 the first surface gradually sinks. In England the material thus 

 yearly brought to the surface varies from seven to eighteen tons 

 per acre, which means an average annual addition of one-tenth 

 to one-sixth of an inch. By this means the surface of the ground 

 is constantly changed, and substances, spread over the ground, 

 in the course of years make their way down into it, forming well- 

 defined layers beneath the surface. 



