Chap. 7. AND DENUDATION. 253 



There were also very many particles of hard 

 mortar, about half of which were well 

 rounded ; and it is not credible that these 

 could have suffered so much corrosion from 

 the action of carbonic acid in the course of 

 only seven years. 



Much better evidence of the attrition of 

 hard objects in the gizzards of worms, is 

 afforded by the state of the small fragments 

 of tiles or bricks, and of concrete in the 

 castings thrown up where ancient buildings 

 once stood. As all the mould covering a 

 field passes every few years through the 

 bodies of worms, the same small fragments 

 will probably be swallowed and brought to 

 the surface many times in the course of cen- 

 turies. It should be premised that in the 

 several following cases, the finer matter was 

 first washed away from the castings, and 

 then all the particles of bricks, tiles and con- 

 crete were collected without any selection, and 

 were afterwards examined. Now in the cast- 

 ings ejected between the tesserse on one of the 

 buried floors of the Roman villa at Abinger, 

 there were many particles (from | to 2 mm. 

 in diameter) of tiles and concrete, which it 



