Ohap. III. BROUGHT UP BY WOB^FS. 137 



beneath the general level of the -field. It 

 should, however, be observed that the thick- 

 ness of the blackish sandy soil, which h-ad 

 been thrown up by the worms above the marl- 

 fragments in the course of 32^ years, would 

 have measured less than 15 inches, if the field 

 had always remained as pasture, for the soil 

 would in this case have been much more 

 compact. The fragments of marl almost rested 

 on an undisturbed sub-stratum of white sand 

 with quartz pebbles ; and as this would be 

 little attractive to worms, the mould would 

 hereafter be very slowly increased by their 

 action. 



We vsrill now give some cases of the action 

 of worms, on land differing widely from 

 the dry sandy or the swampy pastures just 

 described. The chalk formation extends all 

 round my house in Kent ; and its surface, 

 from having been exposed during an immense 

 period to the dissolving action of rain-water, 

 is extremely irregular, being abruptly fes- 

 tooned and penetrated by many deep well- 

 like cavities.* During the dissolution of the 



* These pits or pipes are still in process of formation. During 

 the last forty years I have seen or heard of five cases, in which a 



