WORM BURROWS. 101 



and it is computed that, all over the 32,000,000 acres of 

 cultivated land in which worms are able to live in Great 

 Britain, ten tons of earth are brought to the surface in each 

 acre by the worms alone every year. Moreover, various 

 acids, called humic, are generated by the digestion as well as 

 decay of the vegetable matter which forms so large a part of 

 their food ; and as these seem to be even more powerful 

 than carbonic acid, they must play an important part in the 

 disintegration of the rocks. Worm burrows being frequently 

 five or six feet long and even longer, some small amount of 

 acid at least must be carried down to these depths, there to 

 act upon the underlying rock or fragments of rock ; and as 

 all the mould is in constant though slow movement, fresh 

 surfaces must be continually exposed to the action both of 

 the humic acids and the carbonic acid of the soil. 



The green streaks, sometimes seen in red marls* or 

 sandstone, are caused by the decay of vegetable matter; 

 humic acids have been formed, and these have robbed the 

 red oxide of iron, to which the marl owes its colour, of 

 some of its oxygen, leaving it green or bluish-green ; and, 

 as worms drag enormous quantities of leaves into the 

 ground as linings for their burrows as well as for food, 

 they must greatly promote the formation of these powerful 

 acids. 



Worms also frequently undermine and even penetrate 

 the walls of old buildings, and by thus causing them to sink 

 have by degrees helped to bury them. No building is safe 

 from their burrowing, it is said, unless its foundations be 

 carried down to a depth of six or seven feet. 



* Marls are a mixture of lime and clay with little or no sand. 



