ACTION OP PLANTS AND ANIMALS 183 



moters in bringing about changes in the structure of the soil, 

 and incidentally accelerating decomposition. The organic matter 

 carried by these creatures into the ground, there to decompose, 

 furnishes organic acids to promote further decay in the material 

 close at hand, and by its downward percolation to attack the 

 still firm rocks at greater depths. Indeed, these numerous chan- 

 nels, through affording easy access of air and surface waters with 

 all their absorbed gases or alkaline salts, may serve indirectly a 

 geological purpose scarcely inferior to that of the joints in 

 massive rocks. (See further under soil modified by plant and 

 animal life.) 



The mechanical agency which has already been referred 

 to as instrumental in bringing about a certain amount of de- 

 composition in silicate minerals, is greatly augmented when such 

 trituration takes place in connection with organic matter. J. T. 

 Buchanan has shown,^ that the mud of sea-bottoms is being 

 continually passed and repassed through the alimentary canals 

 of marine animals, and that in so doing the mineral matter not 

 merely undergoes a slight amount of comminution and consequent 

 decomposition, but a chemical reduction takes place whereby 

 existing sulphates are converted into sulphides. Such sulphides 

 and the metallic constituents of the silicates and other compounds 

 particularly those of iron and manganese, would on exposure be- 

 come converted into oxides. It is through these agencies that he 

 would account for the presence of sulphur in marine muds, and 

 the variations in color, from shades of red or brown to blue and 

 gray, in the former the iron occurring as oxides, while in the lat- 

 ter it exists as a sulphide. Of course either form may be more or 

 less permanent according as the mud may be devoid of animal 

 life, or protected from oxidizing influences. These reactions, 

 being subaqueous, are somewhat beyond the scope of the present 

 work, but are nevertheless not without interest in this connection. 



It is further to be noted that the solvent and general chemical 

 activity of water is often greatly augmented by the salts and 

 acids it acquires through the decomposition of various minerals 

 with which it comes in contact. Through the decomposition of 

 iron pyrites there may be formed free sulphuric acid, or through 

 the decomposition of a feldspar, carbonates of the alkalies, any 

 of which, when in solution, are more energetic factors in pro- 



^On the Occurrence of Sulphur in Marine Muds, Proc. Koyal Soc. of 

 Edinburgh, 1890-91. 



