24 AGRICULTURE. 



— lakes of ice formed by the union of many val- 

 ley glaciers — which occupies an area of thirty by 

 seventy miles, and which is covered along its 

 southern border with morainic soil and great 

 forests ; and (c) continental glaciers, covering 

 vast tracts, comprising hundreds of thousands of 

 miles, like those in Greenland and the antarctic 

 land. 



(lo) Icebergs. — When glaciers enter the sea 

 fragments are broken off by the tide — some of 

 them hundreds of feet in depth and more than 

 a mile in diameter — and float thousands of miles 

 before they melt and deposit immense quantities 

 of rock. 



It is evident, then, that the disintegration and 

 transportation of the loose material of the earth's 

 surface by the various forms of water vayy 

 greatly under varying eonditions. 



The chemical action is more rapid in warm, 

 moist countries where vegetation is abundant, 

 while the great variations of heat and cold in 

 the temperate regions, and the powerful frosts 

 in the arctic, render mechanical action more po- 

 tent and swift. 



Again, this work differs in its usefulness to the 

 agriculturist. Sometimes a mantle of loose, 

 workable material is deposited where a short 

 time before the solid rock reached the surface, 

 or great quantities oi organic matter may be 

 deposited which decay and enrich hitherto un- 



