4 68 POPULAR SCIENCE MONTHLY. 



example before us this fact is apparent, and affords the basis for an- 

 other line of reasoning by which all such stratified deposits, however 

 great their magnitude, are to be referred to the same source — namely, 

 stream-transported materials derived from a decaying and wasting 

 land surface, laid down in water under the influence of gravity. 



We have now arrived at a most important and far-reaching gen- 

 eralization so far as the work performed by running water is con- 

 cerned, and its action in filling our lakes and ponds; and we have 

 learned by observation on a small scale the means by which such 

 deposits may be recognized. Let us apply these means of recognition 

 to the phenomena shown by our large rivers and the more enduring 

 oceans into which they drain. In the same manner that we have 

 studied the little pool and larger lake, we will look into the work 

 done by the great waterways of our continents, selecting as a type of 

 such streams the mighty Mississippi. Careful measurement has 

 shown that this river annually transports two hundred million tons 

 of sediment mechanically suspended. What becomes of this enor- 

 mous quantity of sand and clay, equal to a cubic mile in a little over a 

 century, as it is swept into the waters of the Gulf of Mexico? For 

 this purpose we have only to visit the region about its mouth to 

 become acquainted with the almost impotent struggles that have been 

 made by our Government during the last fifty years in an effort to 

 keep the river below New Orleans, in part at least, confined to its 

 present channels; and to study the chart of that portion of the Gulf 

 coast prepared by the United States Coast and Geodetic Survey (see 

 Fig. 6). We have not forgotten the little lobes; their method 

 of growth, and the general form of our first-seen delta, shown 

 in Fig. 3. In viewing the phenomena at the mouth of the Missis- 

 sippi, it is no longer necessary for our present purposes to make a 

 detailed study, since it will become apparent at once that the river is 

 doing the work on a larger scale typified by the performance of the 

 tiny stream flowing into its temporary pool. In place of the little 

 delta with its still smaller lobes, the Mississippi has deposited at its 

 mouth an enormous delta, thousands of square miles in area, and its 

 bifurcating arms may be seen building out several scallops for miles 

 into the waters of the gulf. For centuries these long lobes have been 

 building in advance of the delta front. The arms gradually become 

 clogged with sediment, a new passage to the ocean is opened on the 

 sides, where deposition will begin at a new point, producing a lobe as 

 before. Situated many miles up the river, it is to-day the great fear 

 of New Orleans that its only navigable arm to the sea will thus be 

 closed to that commerce upon which the life of the city depends. 



Only a portion of the sediment brought in by the river goes to 

 form its delta; a large part of the finest material, such as clay, is 



