AQUEOUS AGENCIES. 33 



rounding plain, and the danger from accidental breakage 

 of the levee is ever greater (Fig. 12). It is said that the 

 river Po, from this cause, now runs above the tops of the 

 houses on the plain. 



4. Deltas. 



?he flood-plain of a river may be divided into two 

 parts, viz., the river-swamp and the delta. The river- 

 swamp is that part of the flood-plain which was land- 

 surface when the river began to run, and has been raised 

 only a little by deposit. The delta is that part of the 

 flood-plain which has been reclaimed by the river from the 

 empire of the sea. The river has dumped sediment into 

 the sea or lake, until it filled it up and made a certain 

 amount of land. This made land is the delta. For ex- 

 ample, Upper Egypt is the river-swamp ; Lower Egypt, 

 from Cairo seaward, is the Delta. The flood-plain of the 

 Mississippi, from the mouth of the Ohio to about Baton 

 Eouge, is river-swamp ; thence to the Gulf it is delta. 



A delta may be otherwise defined as an area of flat land 

 at the mouth of rivers, usually of more or less triangular 

 shape, over which the river runs by inverse ramification, 

 emptying by many mouths. The point where the river 

 commences to divide is the head of the delta. The area 

 of some deltas is very great. The delta of the Nile is 

 10,000 square miles, the delta of the Mississippi is 14,000 

 square miles, and the common delta of the Ganges and 

 the Brahmapootra is 20,000 square miles. The form 

 of the Mississippi delta is very irregular. It runs out 

 into the Gulf as a narrow tongue fifty miles long, and 

 separated from the Gulf only by low, narrow embank- 

 ments, which are continuations of the natural levees 

 (Fig. 13). 



Deltas are not formed by all rivers, but only by those 

 which empty into tideless, or nearly tideless, waters. 

 Streams running into pools, ponds, lakes, and. rivers run- 



LE CONTE, GEOL. 3 



