292 DENUDATION OF THE LAND. Chap. VL 



two inches in 10 years. Therefore in so long 

 a period as 2000 years, a large amount of 

 earth will have been repeatedly brought to 

 the surface on most old embankments and 

 tumuli, especially on the talus round their 

 bases, and much of this earth will have been 

 washed completely away. We may therefore 

 conclude that all ancient mounds, when not 

 formed of materials unfavourable to worms, 

 will have been somewhat lowered in the 

 course of centuries, although their inclina- 

 tions may not have been greatly changed. 



Fields formerly ploughed. — From a very 

 remote period and in many countries, land 

 has been ploughed, so that convex beds, 

 called crowns or ridges, usually about 8 feet 

 across and separated by furrows, have been 

 thrown up. The furrows are directed so as 

 to carry off the surface water. In my 

 attempts to ascertain how long a time these 

 crowns and furrows last, when ploughed land 

 has been converted into pasture, obstacles of 

 many kinds were encountered. It is rarely 

 known when a field was last ploughed ; and 

 some fields which were thought to have been 

 in pasture from time immemorial were after- 



