30 



AQUEOUS AGENCIES. 



run out to sea for more than fifty miles, confined only by narrow strips 

 of land, the continuation of the natural levees. These marginal ridges 

 are continued as submarine banks even much beyond the present 

 mouths of the river. The rate of advance of the Nile delta seems to be 

 much slower. 



Age of River-Deposits. — The age of river-swamp deposits may be 

 estimated by determining their absolute thickness and their rate of 

 increase. The river Nile is peculiarly adapted for estimates of this 

 kind, because we have on its alluvial deposits the seat of the oldest 

 civilization and the oldest known monuments of human art. These 

 monuments, the ages of which are approximately known, are many 

 of them more or less buried in the river-deposit. At Memphis, the 



Fig. 22.— Ideal Section of Delta and Submarine Bank. 



foundation of the colossal statue of Rameses II, over 3,000 years old, 

 was found in 1854, buried about nine feet in river-deposit* This 

 makes the rate of increase of the deposit three and a half inches per 

 century. Experiments at Heliopolis bring out nearly the same result. 

 The whole depth of the alluvial deposit at Memphis was found to be 

 about forty feet, which, at the above rate, would make the age of the 

 deposit at this point about 13,500 years. But this all belongs to the 



human epoch, for bricks have 

 been found beneath the lowest 

 part. The alluvial deposit of 

 the Nile is much thicker at 

 some points than forty feet; 

 but, on the other hand, the rate 

 of increase for different places 

 is probably variable. 

 •' The age of a delta is usually 



estimated by dividing the cubic 

 contents of the delta by the 

 annual mud-discharge. The 

 cubic contents of the delta are 

 estimated by multiplying the 

 superficial area by the mean 

 depth. The mean depth of 

 the Mississippi Delta, as deter- 

 mined by borings, is taken by Mr. Lyell as 528 feet, the superficial area 



Fig. %. — Delta and Submarine Bank. 



Philosophical Magazine, vol. xvi, p. 225. 



