126 THEORY OF THE EARTH. 



century. At Cairo, before it is judged sufficient 

 for the purpose of irrigation, it must exceed, by 

 three feet and a half, the height which was neces- 

 sary in the ninth century. The ancient monu- 

 ments of this celebrated land have all their bases 

 more or less buried in the soil. The mud left by 

 the river even covers, to a depth of several feet, 

 the artificial mounds on which the ancient towns 

 were built *. 



The delta of the Rhone is not less remarkable 

 for its increase. Astruc gives a detailed account 

 of it in his Natural History of Languedoc ; and 

 proves, by a careful comparison of the descriptions 

 of Mela, Strabo and Pliny, with the state of the 

 places as they existed at the commencement of 

 the eighteenth century, taking into account the 

 statements of several writers of the middle age, 

 that the arms of the Rhone have increased three 

 leagues in length in the course of eighteen hun- 

 dred years ; that similar additions of land are 

 made to the west of the Rhone ; and that a num- 



* See M. Girard's Observations on the valley of Egypt ; 

 and on the secular increase of the soil which covers it, in the 

 great work upon Egypt, and Mod. Mem. t. ii. p. 343. On 

 this subject we may further observe that Dolomieu, Shaw, 

 and other respectable authors, have estimated these secular 

 elevations much higher than M. Girard. It is to be lament- 

 ed, that nowhere has it been tried to examine the depth of 

 these deposits over the original soil, or the natural rock. 



