4E» 



CHAPTER XVI. 



CHARACTERS AND LITHOGENESIS OF THE BIOCLASTIC ROCKS. 



Bioclastic rocks consist of fragments of older rocks which have 

 been broken by the mechanical activities of organisms. Among 

 these man is the most active, and he is undoubtedly the greatest 

 producer of bioclastic rocks. To distinguish this type the name of 

 artificial elastics is applied to it. and as examples concrete and other 

 artificial stone may be mentioned. These need not be more fully dis- 

 cussed, though their variety is great and their characters manifold. 



There are also other organisms which in a more or less efifec- 

 tive way render rocks clastic and so furnish the material from 

 which new rocks may be formed. Chief among these are the great 

 herds of vertebrates of the plains and steppes and the even larger 

 animals of former geological periods. 



Pechuel-Loesche (y.83^') describes the destruction of the land 

 surface through the immense herds of cattle which he had wit- 

 nessed in Herero land, German Southwest Africa. He says in ef- 

 fect: "In extensive manner these animals aid in the leveling of 

 many land areas. As the dryness increases, the herds of grazing 

 cattle become more numerous around the last of the sparsely dis- 

 tributed water bodies. Thousands and tens of thousands of the 

 large and the small animals overrun for miles the surrounding 

 country for days, weeks and months. Through countless hoof- 

 beats the ground is loosened, and so furnishes enormous masses of 

 dust, while at the same time all inequalities are trampled down and 

 destroyed. The inclined surfaces would be furrowed by numerous 

 rain-water gullies, if these were not constantly destroyed by the 

 hoofs of the roaming animals, and if it were not for the fact that 

 the rain water is constantly guided along the paths formed by the 

 animals going to and from the water in long lines ranged one be- 

 hind the other. Furthermore, the cover of dust prevents to an as- 

 tonishing degree the penetration of the short, heavy downpour of 

 rain into the deeper strata." 



Where herds abound, the surface of the country is a compara- 



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