PRESENT CHANGES IN THE EARTH. 91 



and attraction, as was fully explained in Part I. But 

 water has a larger agency in producing the changes in 

 the earth than air. Through the influence of heat it is 

 carried up constantly into the atmosphere in evapora- 

 tion, and then falls in rain, snow, etc., and in seeking its 

 level in obedience to attraction, effects a large portion of 

 these changes. The influence of heat is seen in the erup- 

 tions of volcanoes and other phenomena, and these indi- 

 cate to some extent the agency which heat had in build- 

 ing up the earth, as do the present effects of moving wa- 

 ter what agency that had in the work. It is these aque- 

 ous and igneous agencies, as they are termed, which, oft- 

 en acting in opposition and sometimes in unison, have 

 for the most part arranged and consolidated the mate- 

 rials of the earth, so as to put it into its present condi- 

 tion ; and therefore it is necessary to view their present 

 operations, that we may understand those which were 

 carried on in the ages that preceded the advent of man, 

 when the earth was being prepared by successive steps 

 to be his habitation. This will be, then, the line of our 

 investigation in the present chapter. 



173. Water Changing the Locality of Materials. What 

 we see in the washings of every shower on slopes and 

 hill-sides exemplifies some of the vast changes which wa- 

 ter is producing on a large scale in the earth. Different 

 kinds of materials are moved according to the different 

 degrees of rapidity with which the water flows. If you 

 look at a mountain torrent you see nothing in its course 

 but large stones, because not only mud and sand, but 

 pebbles of considerable size, are carried along by the 

 force of the water. If you go farther on, where the 

 stream is less rapid, you will find the pebbles, and if the 

 rapidity lessens as you follow the stream, there will be 

 sand on its bottom ; and when the water moves on slow- 

 ly through a plain, you will find the sediment deposited 

 to be mud. The explanation of this sorting out of ma- 

 terial by moving water has been given in 193, Part I., 



