NATURE AND FORMATION OF SOILS. 35 



of earthworms has both a mechanical and a 

 chemical effect. The burrows may extend sev- 

 eral feet under ground, and are connected with 

 each other by underground tunnels, so that the 

 soil is thoroughly exposed to the chemical action 

 of gases and acids of the air and water. The 

 muscular gizzard grinds the stony particles swal- 

 lowed by the worm, making them finer and 

 more succeptible to the humic acids, the gener- 

 ation of which is probably hastened during the 

 digestion of the vegetable mould and half- 

 decayed leaves, upon which the worm feeds. 



(2) ANIMAL ACCUMULATIONS. Calcareous 

 Deposits. " The sea is constantly receiving 

 from the land materials in solution, the most 

 important of which are the carbonate and sul- 

 phate of lime. Many classes of marine animals 

 extract the calcium carbonate (CaCO 3 ) from the 

 sea-water and form it into hard parts, either as 

 external shells and tests or as internal skeletons. 

 There is also good reason to believe that some, 

 at least, of these organisms are able to convert 

 the sulphate into the carbonate." In shallow 

 seas, where the conditions of warmth and food- 

 supply are favorable, animal accumulations are 

 developed on a large scale. The most impor- 

 tant of these accumulations are those of the 

 corals,* echinoderms, and mollusks, 



* Scott's Geology, pp. 165-170, 



