Evolution of the Earth. 81 



vegetable life, there were no stratified rocks ; — the earth 

 was a vast cooling cinder when the forces began to operate 

 which have produced the manifold forms of life and beauty 

 that now diversify its surface. The great agencies which 

 have effected this evolutionary process, may be classed as 

 atmospheric, aqueous, igneous and organic. The former 

 two, the atmospheric and aqueous, have been leveling and 

 apparently destructive ; tearing down rocks and mountains, 

 creating sand and soil, filling the valleys, and making the 

 rough places of the earth smooth, softening its original 

 hardness. The latter, the igneous and organic, have been 

 upbuilding and creating agencies, lifting islands and con- 

 tinents out of the ocean beds, and elevating mountain 

 ranges, thus modifying the climate and productiveness of 

 favored localities ; though igneous agencies have sometimes 

 operated destructively as well as constructively. 



Action of the Atmospheric and Aqueous Agencies. 

 Up to the time when our history opens, the igneous agen- 

 cies, together with the great cosmic force of gravity, had 

 been mainly influential in moulding the earth, and shaping 

 its external form. With the condensation of the watery 

 vapors, and their descent upon the earth, the aqueous and 

 atmospheric forces became for a time the dominant agencies 

 in effecting geological transformations. As the surface of 

 the earth was much more level than at present, the con- 

 tinents and islands were smaller and fewer in number than 

 they are now ; the seas were shallow, but a much greater 

 proportion of the earth's surface was covered with water. 

 Let us now consider how the atmospheric and aqueous agen- 

 cies would operate in transforming the exposed areas of 

 land, and what processes would go on in the depths of 

 the oceans and lakes, and along the river-courses. 



If we go over to the Palisades on the west bank of the 

 Hudson River, we shall observe, as we know, several miles 

 of almost perpendicular cliffs, reaching a height at one or 

 two points of more than five hundred feet. All along the 

 foot of the cliffs, we find masses of broken rock and debris, 

 growing from year to year by the falling of portions of the 

 rock which are loosened and separated by the action of 

 water, snow and frost. This action goes on most ra])idly in 

 the winter and the spring. The water and snow are blown 

 against the cliff, permeating the crevices in the rocks. 

 Freezing and expanding, the ice splits the solid mass asun- 



