MECHANICAL AGENCIES OF WATER. 73 



tances from their parent rock. Also such sheets packed together in 

 large masses, and driven ashore by river and tidal currents, and chafed 

 back and forth by waves, produce effects on the shore-rocks somewhat 

 similar to the scoring, polishing, and even the roclies moutonnees of 

 glacier-action. On a rising or on a subsiding coast such scorings and 

 polishings may extend over wide areas, and thus simulate true glacial 

 action. These effects are well seen on the shores of the St. Lawrence 

 Eiver and G-ulf. 



The importance of the study of ice-agencies will be seen when we 

 come to explain the phenomena of the Drift or Glacial period. 



Comparison of the Different Forms of the Mechanical Agencies of 



Water. 



Rivers and glaciers are constantly cutting down all lands, bearing 

 away the materials thus gathered, and depositing them on the sea- 

 margins. Acting alone, therefore, their effect must be to diminish the 

 height and to extend the limits of the land. Ocean agencies, on the 

 other hand, by tides and currents bear away to the open sea the mate- 

 rials brought down from the land, and thus tend to prevent marginal 

 accumulations ; and by waves and tides constantly eat away the coast- 

 line, and thus strive to extend the domain of the sea. Thus, while 

 river and ocean agencies are in conflict with one another at the coast- 

 line, the one striving to extend the limits of the land, and the other 

 of the sea, yet they co-operate with each other in destroying the in- 

 equalities of the earth's surface, and are therefore called leveling agen- 

 cies. Moreover, it is evident that the erosion of the land and the fill- 

 ing up of the seas are correlative, and one is an exact measure of the 

 other. Xow, we have seen (page 11) that the probable rate at which 

 all continents are being cut down by rivers is about one foot in 4,500 to 

 5,000 years. But since the ocean is about three times the extent of 

 the land, this spread evenly over the bottom of the sea would make a 

 stratum about four inches thick. Therefore, we conclude that, neg- 

 lecting the destructive effects of waves and tides on the coast-line, 

 which, according to Phillips,* are small in amount compared with gen- 

 eral erosion of the land-surface, we may say that stratified deposits are 

 now forming, or the ocean-bed filling up, at the average rate over the 

 whole bottom of about four inches in 5,000 years. 



* Phillips, Life on the Earth, p. 131. 



