LAN1> AM) WAl'KU. 27 



As the process of cooling- went on, the moisture of the 

 atmosphere became condensed ; and, falling as rain, cov- 

 ered the earth's surface with water. 



Out of this })rimitive earth, this crust of mineral sub- 

 stances, and the water and atmosphere surrounding it, 

 and out of the ]>lants and bodies of aninuils that have 

 lived and died u])on its surface, have been formed the 

 soil and rocks of our present earth. 



The Age of the Earth. — The processes Ity whicli this 

 great change has been brought about have been, for the 

 most part, silent and gradual. Many of them are still 

 going on. Our earth, with the variety of sul)stances 

 upon it, is not to be considered as a thing completed, but 

 as constantly undergoing changes in the great workshop 

 of Nature. 



The period of time that has elapsed since the first 

 solid crust was formed must be exceedingly long. Some 

 of the lowest estimates made by careful students have 

 been from fifteen to twenty million years. 



Continents. — It is a common principle in nature that 

 heating a body causes it to expand, and cooling causes 

 it to contract, or become smaller. Now, it is impossible 

 for a spherical body, Avith a solid surface, to become 

 smaller without forming upon its surface dents and 

 I'idges, or depressions and elevations. As the ancient 

 earth became still cooler and smaller, after having first 

 formed a crust, it was natural that this crust should be- 

 come irregular in shape, producing low and high places. 

 The water naturally sank into the lower places, leav- 

 ing the elevated regions as continents. 



The continents that thus fii-st ai)peared above water 

 were very small, and gradually increased in size, extend- 

 ing their coasts as the continued shrinking of the crust 



