86 DOCTRINE OF EVOLUTION 



miles or so to open out into Lake Ontario. The water 

 passing over the brim of the falls wears away the edge 

 at a rate which varies somewhat according to the harder 

 or softer consistency of the rocks, but which, since 

 1843, has averaged about 104 inches a year. Knowing 

 this rate, the length of the gorge, and the character of 

 the rocky walls already carved out, the 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 agree- 

 ment 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 sohdified 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 accu- 

 rately measured, and a satisfactory determination can 

 also be made of the total amount of water carried by in a 



