96 Evolution of the Earth. 



down to soils ; soils are carried down and deposited as sed- 

 iments; and sediments consolidate as rocks — the same 

 materials being worked over and over again, passing through 

 all these changes many times in the history of the earth. 

 In a word, there was a time when it was thought that the 

 earth, with substantially its present form, configuration and 

 climate, was made at once out of hand, as a fit habitation 

 for man and animals. Now we know that it has been chang- 

 ing, preparing, becoming what it is by a slow process, 

 through a lapse of time so vast that the mind sinks ex- 

 hausted in the attempt to grasp it. It has become what it 

 is now by a process of evolution. We may, therefore, con- 

 fidently generalize — we may assert without fear of con- 

 tradiction that all inorganic forms, without exception, have 

 originated by a process of evolution."* 



Let us briefly recapitulate the forces which have been 

 active in the evolution of the earth : — 



I. Igneous. 



a. The formation of the primitive rock-crust by cooling. 



b. The formation of crystalline rocks under pressure. 



c. Volcanic action, producing pumice, lava, basalt, por- 



phyry, tuff, etc. 



d. Action of geysers, or hot springs. 



To these should be added those chemical processes, the 

 operation of which was largely determined by conditions of 

 heat and pressure. 



II. Atmospheric. 



a. Direct influence of wind-pressure. 



b. Disintegrating influence of snow and ice. 



c. Glacial action. 



d. Precipitation of atmospheric gases. 



e. Expansion and contraction by changes in temper- 



ature. 



III. Aqueous. 



a. Direct action of rain on the rock. 



b. Action of rivers in distributing sedimentary matter. 



c. Disintegrating action of ocean-waves. 



d. Operation of large bodies of water in facilitating 



the formation of sedimentary deposits. 



*" Evolution and its Relation to Religious Thought," by Professor Joseph 

 Le Conte. D. Appleton & Co. 



