CHAP. II. THE COLLAPSE OF OLD BURROWS. 121 



tlie whole ground would be first thickly 

 riddled with holes to a depth of ahout ten 

 inches, and in fifty years a hollow unsup- 

 ported space, ten inches in depth, would be 

 left. The holes left by the decay of succes- 

 sively formed roots of trees and plants must 

 likewise collapse in the course of time. 



The burrows of worms run down perpen- 

 dicularly or a little obliquely, and where the 

 soil is at all argillaceous, there is no difficulty 

 in believing that the walls would slowly flow 

 or slide inwards during very wet weather. 

 When, however, the soil is sandy or 

 mingled with many small stones, it can 

 hardly be viscous enough to flow .inwards 

 during even the wettest weather; but another 

 agency may here come into play. After 

 much rain the ground swells, and as it cannot 

 expand laterally, the surface rises ; during dry 

 weather it sinks again. For instance, a large 

 flat stone laid on the surface of a field sank 

 3*33 mm. whilst the weather was dry between 

 May 9th and June 13th, and rose 1*91 mm. 

 between September 7th and 19th of the same 

 year, much rain having fallen during the latter 

 part of this time. During frosts and thaws 



