AQUEOUS AGENCIES. 31 



Nile is the whole land of Egypt, for without the Nile the 

 whole of Egypt would be a desert. Egypt is literally the 

 daughter of Nilus. The flood-plain of the Mississippi 

 extends from the mouth of the Ohio Eiver to the Gulf 

 its area is 30,000 square miles. 



Now, since great rivers always rise in mountain-regions, 

 and since the general rain-erosion in such regions is very 

 great, it is evident that in flood seasons they gather abun- 

 dant sediment, and, when these muddy waters overflow, 

 the checking of velocity causes abundant deposit all over 

 the flood-plain. With every flood this deposit is renewed, 

 and the stratum becomes thicker. Thus, the level of the 

 flood-plain is built up by sedimentary deposit, without 

 limit. In the Mississippi Eiver the flood-plain deposit is 

 about fifty feet thick. In the Nile it is forty to eighty 

 feet thick. 



Time. On the flood-plain of the Nile stand the oldest 

 monuments of civilization in the world. One of these 

 (the statue of Rameses II), supposed to be 3,000 years 

 old, has been covered about the base with sediment nine 

 feet deep. The whole thickness of the Nile sediment at 

 this point is forty feet nine feet in 3,000 years would 

 make forty feet in 13,330 years as the age of the Nile 

 deposit. This is, of course, but a rough estimate. The 

 rate may not have been uniform. But in any case the 

 whole time belongs to the present geological epoch. 



Levees, Natural and Artificial. In rivers which 

 regularly flood their plains we always find a sort of em- 

 bankment on either side near the river, higher than the 

 rest of the flood-plain, and consisting of coarser material. 

 This is called the natural levee (Fig. 11, /, /). When the 

 river is at full flood, e } e, the whole flood-plain is covered, 

 but at half flood d, d, d, it is often divided into three 

 streams, viz., the river channel and the back swamp on 

 either side. The cause of the natural levee may be explained 

 thus : The whole flood-plain is covered with water moving 



