Recently an examination was made into the waters of nineteen important rivers, to discover what quantity of material was carried by each. The result of this examination was somewhat startling. We have seen already how many cubic miles of water a river may give over yearly to the great deep. It was found that, on an average, these rivers may be reckoned to give over also, _with each cubic mile of water_, more than _seven hundred and sixty thousand tons of material_, torn from the land. These enormous collections, together with vast quantities which the waves have dug and wrenched from beaches and cliffs, are dropped to Ocean’s floor. Not indeed generally to her deepest parts. So far as we yet know, the Ocean does not build continents in those black profound abysses, miles deep, flat-floored, calm, unchangeable. No traces have been there discovered of such strata as form the Continental parts of Earth’s Crust. Ocean’s main work, as builder, goes on in “The Transitional Area;” and most so in that part of it which is called “The Continental Shelf,” where the depth does not exceed about six hundred feet. “Transitional” regions are so named because it is believed that some of them may once have been Continents, that others of them may yet become Continents. There was a time, in the long past, when ocean-waters flowed over all those parts of Earth which now are known to us as dry land. There may be a time, in the far future, when areas of comparatively shallow water will become dry land. Through ages the task of taking materials from the land for building purposes has been carried on by the Ocean, and it goes on still. The Continents are being steadily worn away; the Islands are becoming continually smaller. Were this to continue always, with no counteracting forces, islands and continents must in the end disappear. It would take a very long time; probably well over six millions of years, at the present rate of demolition. Still, however long deferred, the end would be sure. But counteracting forces exist. CHAPTER XII. ABOUT THE LONG PAST “The cold and silent past.”--WHITTIER. “Thou coveredst it with the Deep as with a garment; the waters stood above the mountains.”--Ps. civ. 6. The very long past--how long no man can say. Some rash attempts have been made to name the numbers of thousands of years which may have elapsed since this or that particular stage in the world’s geological history. Such reckonings can possess little or no true value. It is better to confess frankly that we cannot fix dates. We only know that certain changes did come about--very long ago. Our Earth has not always been as she is now. She has altered greatly. She is altering still. Development has gone on through the ages. Development goes on still.