87 



not caused by lines of underground drainage, to which 

 they are transverse in general. 



True lines of underground drainage may be found in 

 the case of folds whose direction is so much inclined to the 

 vertical as to receive at their upper lips the surface drain- 

 age, and conduct it back to the bottom of the fold, where if 

 the rock be soluble it forms pipes [see PI. '\. fig. 5.) 



Pipes were never produced by the same means which 

 produced Trail and Underplight, the tendency of which 

 was to obliterate them. Pipes were either pre-existing or 

 subsequent to the latter events. In the case of pre-existing 

 pipes, the contents of which have been bent over and 

 carried away horizontally, the relationship is obvious, as 

 in the accompanying cut (in the upper gravel sheet at 

 Greenhithe). But true piping of a subsequent date is very 

 difficult to determine. 



Note.— In this figure the radiant lines shew the cracks in the loam 

 of the core due to flexure. 



In soluble rocks, especially chalk, the removal of the 

 lime has assisted in letting the loose gravels down, the 

 line which separates the chalky trail from the decalcified 

 trail is mostly sharp and easily distinguished however ; 

 I think that this solution of the lime has had little or 

 nothing to do with the cause of the peculiar forms exhibited 

 in the curves and outlines which I figure. 



True gullies or lines of drainage on the glaciated 

 surface may of course have been developed, but they 



