It is by these agencies that the hills and the mountains are graded down, and the _detritus_ deposited in the valleys and in the sea; thus are valleys enriched and broadened, vast plains and deltas created, and continents enlarged. When the present hills and mountains have been reduced to plains, and the fertility of the soil exhausted, it is quite probable that another grand upheaval of the earth's foundations will occur,--the birth-power by which new hills and mountains are lifted up, and continents changed to ocean-beds, and ocean-beds to continents. It is these mighty changes and exchanges that prepare the way, and fit the earth for the production of higher orders of plants and animals, and perhaps a higher order of man. In the course of unknown ages, Nature has enriched and extended the valley of the Nile hundreds of miles into the sea, by transporting thither the pulverized wealth of the Abyssinian mountains. Thus fertilized, Egypt has for many thousands of years sustained a dense population. Very justly has she been called not only the cradle of mankind, but the granary of the world. In like manner, the Ganges transports from the interior of India a sufficient amount of sediment annually to cover a township five miles square to the depth of ten feet, and by this means has extended the land hundreds of miles into the ocean. The Hoang-Ho, a river of China, by its deposits of alluvium in the sea has added an entire province to that country, comprising an area of ninety-six thousand square miles. Indeed, all rivers are tributaries to the sea, and all seas tributaries to the rivers. This exchange is effected mainly by the rains and the snows, the exhalations and the waterspouts. The clouds are but common carriers; this commerce is therefore a matter of mutual interest, and grows out of the positive necessities of sea and land. Though the elements appear to move in conflict, they really move in perfect harmony, and bring order out of seeming confusion. In executing a gigantic work, no river has excelled the Mississippi. This "Father of Waters" has distinctly indicated in the record of his career the prehistorical age of the world, and the equally prehistorical advent of man. In his "march to the sea" he has left enduring landmarks, and with his battle-axe notched centuries long lost in the mighty past. The land which this majestic river has formed, by depositing sediment in the Gulf of Mexico, comprises an area of thirty thousand square miles. This deposit or delta has a depth exceeding one thousand feet; and the period required for its accumulation has been estimated by Mr. Lyell, the renowned geologist, at one hundred thousand years.