WATER AS A MECHANICAL AGENT. 



19T 



tions over tlie various parts thus carried forward, along with the aid of 

 encroaching vegetation, a large portion of a delta may become emerged. 

 More than two thirds of the Mississippi delta in the ordinary state of the 

 river are above water ; and over this part are plantations of rice, sugar, and 

 cotton, and cypress forests. The area of actually productive land within it 

 is 22,920,320 acres; of reclaimable land, 35,813 square miles. But if the 

 river were unrestrained by levees, the highest floods would fill the alluvial 

 basin and make a sea 600 miles long, 60 miles in mean width, and 12-|- feet 

 in mean depth. (C. G. Forshey, 1873.) The force of the flood-waters of the 

 Mississippi is so great, and the amount of transported detritus so large, that 

 the stream pushes out its long arms into the Gulf, by its method of deposit- 

 ing load after load ; and it is still continuing its elongations at the extremities 

 of the passes. 



184. 



Delta of the Mississippi. 



The shallow waters within one to three miles of the main channel at the mouth of the 

 Mississippi River (see map) are dotted with what are called mud-lumps, — convex or 

 low conical elevations, sometimes 100 feet or more in diameter, showing their tops at 

 the surface. They originate in upheavals of the soft but tough bottom. Once formed, 

 they discharge mud from the top, which gives to the material of the low cone the structure 

 of a volcanic cone, the successive laj'ers being, however, of mud, and but a fraction of an 

 inch thick. They finally collapse ; and then the cavity of the cone sometimes becomes the 

 site of a pool of salt-water, like the lake in an extinct volcano. They are formed, accord- 

 ing to Professor E. W. Hilgard (from whose excellent description in the American 

 Journal of Science^ 1871, the facts here given are cited, and who adopts, in the main 



