CHAP. V. AND DENUDATION. 259 



consider the evidence above given, there can 

 be little 'doubt that the fragments, which serve 

 as millstones in the gizzards of worms, suffer, 

 when of a not very hard texture, some amount 

 of attrition; and that the smaller particles in 

 the earth, which is habitually swallowed in 

 such astonishingly large quantities by worms, 

 are ground together and are thus levigated. 

 If this, be the case, the " terra tenuissima," 

 the "pate excessivemerit fine," of which the 

 castings largely consist, is in part due to the 

 mechanical action of the gizzard ; * and this 

 fine matter, as we shall see in the next chapter, 

 is that which is chiefly washed away from the 

 innumerable castings on every field during 

 each heavy shower of rain. If the softer stones 

 yield at all, the harder ones will suffer 

 some slight amount of wear and tear. 



The trituration of small particles of stone 



* This conclusion reminds me of the vast amount of extremely 

 fine chalky mud which is found within the lagoons of many 

 atolls, where the sea is tranquil and waves cannot triturate the 

 blocks of coral. This mud must, as I believe (' The Structure and 

 Distribution of Coral-Reefs,' 2nd edit. 1874, p. 19), be attributed 

 to the innumerable annelids and other animals which burrow 

 into the dead coral, and to the fishes, Holothurians, &c., which 

 browse.on the living corals. 



T 



