140 THEORY OF THE EARTH. 



by dikes, at the height of twenty or even thirty feet 

 above the level of the land. 



M. Wiebeking, director of bridges and highways 

 in the kingdom of Bavaria, has given an excellent 

 memoir upon this subject, so highly important to be 

 known and understood thoroughly, both by the 

 people and the government, in all countries liable 

 to these changes. In this memoir he has demon- 

 strated that all rivers are continually elevating the 

 levels of their beds, more or less, according to cir- 

 cumstances. 



This formation and increase of new grounds by 

 alluvial depositions, proceeds with as much rapidi- 

 ty along the coasts of the North Sea as on those 

 of the Adriatic. These additions can be easily 

 traced in Friesland and Groningen, where the 

 epoch of the first dikes, constructed by the Spanish 

 governor, Gaspard Robles, is well known to have 

 been in 1570. An hundred years afterwards, the 

 alluvial depositions had added in some places three 

 quarters of a league of new land on the outside of 

 these dikes: And the city of Groningen, partly 

 built upon the ancient soil which has no connexion 

 with the present sea, being a calcarious formation, 

 in which the same species of shells are found as in 

 the coarse limestone formations near Paris, is only 

 six leagues from the sea. Having been upon the 

 spot, I can give my testimony to the facts already 

 so well stated by M. Deluc in his Letters to the- 



