Here is a fine piece of bottom land, one of those "banks" where the rivers keep "checking accounts" for the farmers and the sea; using pebbles for currency, as explained in this chapter.] HOW THE RIVERS PLACE PEBBLES ON DEPOSIT The rivers make loans to the soil by depositing pebbles in the broad bottom-lands along their banks, and then draw interest by carrying along to other lands, from time to time, some of the fine rich soil these pebbles help make by their decay. And the river does this in regular banking style, "checking out" the pebbles from time to time, and then depositing other pebbles in their places. Take the banks and bottom-lands of the Mississippi River, for example. It has been estimated that it requires about 40,000 years for a pebble to make the journey to the Gulf from the mountains of a tributary stream where it was first broken from the rock as a sharp fragment. The first part of the journey in the mountains is over steep down grades, and so is comparatively fast, but as the river gets farther from the mountains, the slope of its bed becomes less and less, the onward movement is slower and slower, and more of the pebbles stop to rest. In times of flood they are carried far away from the regular channel and spread over the wide flood-plain of the river. Then, as the flood goes down, they are left buried there under a coating of mud. So buried, they decay and enrich the soil. Then the next flood that comes along sweeps the pebbles with it--checks them out of the bank--but at the same time carries away not only some of the soil richness which these pebbles helped to make but the soil material made by the decay of the vegetation these pebbles thus helped to grow, such as the roots and blades of wheat and corn and stubble and chaff left in the fields. That's the interest on the loan. Then, when the flood subsides, the pebbles are again deposited farther along in the river's course, but meanwhile the same flood has brought fresh deposits of pebbles from up-stream, and these are left in place of those taken away. RIVER BANKING AND HUMAN CIVILIZATION