LITHOGENESIS OF THE BIOCLASTTC ROCKS 695 



Plants also are destroyers of rocks, though their work is nor- 

 mally very slow. Lichens growing on smooth rock surfaces will 

 eventually roughen them by appropriating some of the material. 

 Roots of higher plants often penetrate into the rock, especially 

 limestones, to an astonishing extent. In sandstones they have been 

 found to penetrate several meters. Growing saplings in fissures 

 tend to disrupt the rock masses. Finally bacteria abound in the 

 upper soil layers {2y2 millions have been estimated in a cubic cen- 

 timeter of soil in the surface layers), and these are active agents 

 in modifying the soil. 



It thus appears that the work of organisms is by no means a 

 negligible factor, and will in the course of time produce important 

 results. Of these organisms man is of course the most important, 

 and it is not going too far to say that on the whole he is the most 

 powerful geological agent at work in modifying the surface of the 

 land. 



BIBLIOGRAPHY XVI. 



1. BRANNER, JOHN C. 1896. Decomposition of Rocks in Brazil. Bulletin 



of the Geological Society of America, Vol. VII, pp. 255-314. 



2. BRANNER, J. C. 1900. Ants as Geological Agents in the Tropics. 



Journal of Geology, Vol. VIII, pp. 151-153. 



3. BRANNER, J. C. 1910. Geologic Work of Ants in Tropical America. 



Bulletin of the Geological Society of America, Vol. XXI, pp. 449-496. 



4. DARWIN, CHARLES. 1883. The Formation of Vegetable Mould. 



D. Appleton & Co., New York, pp. 1-313. 



5. DAVISON, CHARLES. 1891. Work Done by Lobworms. Geological 



Magazine, 3d Ser., Vol. VIII., pp. 489-493. 



6. PASSARGE, SIEGFRIED. 191 1. Die pfannenformigen Hohlformer der 



siidafrikanischen Steppen. Petermann's Mittheilungen LVII, pt. ii, pp. 



130-135- 



7. PECHUEL-LOESCHE. 1884. Das Ausland. 



8. SHALER, NATHANIELS. 1892. Eflfects of Animals and Plants on Soils. 



In "The Origin and Nature of Soils, " U. S. Geological Survey, 12th Annual 

 Report, pt. I, pp. 219-345 (268-287). 



