length of time necessary for its production can be safely estimated.  It is about 30,000 to 40,000 years, not a long period when the whole history of the earth is taken into account.  A similar length of time is indicated for the recession of the Falls of St. Anthony, of the Mississippi River, an agreement that is of much interest, for it proves that the two rivers began to make their respective cuttings when the great ice-sheet receded to the north at the end of the Glacial epoch.

What has become of the masses washed away during the formation of these gorges?  As gravel and mud and silt the detritus has been carried to the still waters of the lower levels, to be laid down and later solidified into sandstone and slate and shale.  All over the continents these things are going on, and indefatigable forces are at work that slowly but surely shear from the surface almost immeasurable quantities of earth and rock to be transported far away.  In some instances it is possible to find out just how much effect is produced in a given period of time, especially in the case of the great river systems.  For example, the mass of the fine particles of mud and silt carried in a given quantity of the water of the Mississippi as it passes New Orleans can be accurately measured, and a satisfactory determination can also be made of the total amount of water carried by in a year.  From these figures the amount of materials in suspension discharged into the Gulf of Mexico becomes known.  It is sufficient to cover one square mile to the depth of 269 feet; in twenty years it is one cubic mile, or five cubic miles in a century.  Turning now to the other aspect of this process, and the antecedent causes which produce these effects, it appears that the area of the Mississippi River basin is 1,147,000 square miles—­about one third of the total area of the United States.  Knowing this, and the annual waste from its surface, it is easy to demonstrate that it will take 6000 years to plane off an average of one foot of soil and rock from the whole of this immense area.  Of course only an inch or a few inches will be taken from some regions where the ground is harder or rockier, or where little rain falls, while many feet will be washed away from other places.  The waters of the Hoang-ho come from about 700,000 square miles of country, from which one foot of soil is washed away in 1464 years.  The Ganges River, draining about 143,000 square miles, carries off a similar depth of eroded materials from its basin in 823 years!  Should we add to the above figures those that specify the bulk of the chemical substances in solution carried by these waters, the total would be even greater.  We know that in the case of the Thames River, calcareous substances to the amount of 10,000 tons a year are carried past London, and all this mineral has been dissolved by rain-water from the chalky cliffs and uplands of England, so that the land has become less by this amount.  Thus we learn that vast alterations are being made