102 Evolution of the Earth. 



are doubtless too small, and imagine, if you can, the time 

 required for the Colorado river to wear its way down to its 

 present bed at the bottom of a canon 5,000 feet, in many 

 places, below the level of the surrounding plain, through a 

 rock harder than that of the Niagara chasm, and without 

 the initiative force of the tremendous cataract, for a distance 

 of six or seven hundred miles ! This is one of the meas- 

 uring sticks by which the estimate of the duration of geolog- 

 ical periods is made. All such estimates, however, must be 

 very imperfect guesses at the truth. The thickness of geo- 

 logical strata varies in different localities. The effect of 

 local disturbances is such as to render it difficult to base an 

 accurate estimate of time on such data as are obtainable. 

 We can only say that immense, practically inconceivable 

 periods of duration were requisite for the production of the 

 observed effects. The present age of the earth has been va- 

 riously estimated at from 25,000,000 to 100,000,000 years 

 — - the latter are Professor Geikie's figures. I suppose we 

 might as well try to imagine one period as the other — both 

 are so long as to be incomprehensible by the human mind. 

 As Professor Le Conte affirms, such a period constitutes a 

 practical eternity. 



In conclusion, let me refer again to Le Conte's inspiring 

 words : — " Until the birth of modern astronomy the intel- 

 lectual space-horizon of the human mind was bounded sub- 

 stantially by the dimensions of our earth : sun, moon and 

 stars, being but inconsiderable bodies circulating at a little 

 distance about the earth, and for our behoof. Astronomy 

 was then but the geometry of the curious lines traced by 

 those wandering fires on the concave blackboard of heaven. 

 With the first glance through a telescope, the phases of Ve- 

 nus and the satellites of Jupiter revealed clearly to the 

 mind the existence of other worlds besides and like our own. 

 In that moment the idea of infinite space, full of worlds 

 like our own, was for the first time completely realized, and 

 became thenceforth the heritage of man. In that moment 

 the intellectual horizon of man was infinitely extended. 

 So also, until the birth of geology, about the beginning of 

 the present century, the intellectual time-horizon of the hu- 

 man mind was bounded by six thousand years. The dis- 

 covery about that time of animal remains, all wholly dif- 

 ferent from those now inhabiting the earth, revealed the 

 existence of other time fauna besides our own ; and the idea 



