156 EARTH FEATURES AND THEIR MEANING 



are carried through by the agency of earthworms, bacteria, and 

 other organisms, and by the action of humus and other acids de- 

 rived from the decomposition of vegetation. The bacteria par- 

 ticularly play a part in the formation of carbonates, as they do 



also in changing 

 the nitrogen of 

 the air into ni- 

 trates which be- 

 come available 



FIG. 164. Diagram to show the varying thickness of as plant food. 

 mantle rock upon the different portions of a hill surface Within the 

 (after Chamberlin and Salisbury). humid ^.^ 



regions ants and other insects enter as a large factor in rock 

 decomposition, as they do also in producing not unimportant 

 surface irregularities. 



How important is the cover of vegetation in retaining the rock 

 mantle and the upper soil layer in their respective positions, as 

 required for agricultural purposes, may be best illustrated by the 

 disastrous consequences of allowing it to be destroyed. Wherever, 

 by the destruction of forests, by the excessive grazing of animals, 

 or by other causes, the mat of turf has been destroyed, the sur- 

 face is opened in gullies by the first hard rain, and the fertile layer 

 of soil is carried from the slopes and distributed with the coarser 

 mantle upon the bottom lands. Thus the face of the country is 

 completely transformed from fertile hills into the most desolate 

 of deserts where no spear of grass is to be seen and no animal food 

 to be obtained (plate 5 A). The soil once washed away is not again 

 renewed, for the continuation of the gullying process now effec- 

 tively prevents its accumulation. 



READING REFERENCES TO CHAPTER XI 

 Decomposition and disintegration : 



GEORGE P. MERRILL. The Principles of Rock Weathering, Jour. Geol., 

 vol. 4, 1896, pp. 704-724, 850-871. Rocks, Rock Weathering, and 

 Soils. Macmillan, New York, 1897, Pt. iii, pp. 172-411. 

 ALEXIS A. JULIEN. On the Geological Action of the Humus Acids, Proc. 

 Am. Assoc. Adv. Sci., vol. 28, 1879, pp. 311-410. 



Corrosion of rocks : 



iC. W. HAYES. Solution of Silica under Atmospheric Conditions, Bull. 

 Geol. Soc. Am., vol. 8, 1897, pp. 213-220, pis. 17-19. 



