EFFECTS OF WEATHER 



and stones of its channel, though this action is not very 

 strikingly shown. But sometimes the rusting or dis- 

 solving action of water is very evident. When it issues 

 from a peat bog, for example, and is consequently highly 

 charged with acid, it will make a very great impression 

 on any limestones it may encounter ; for as any school- 

 boy knows who has ever put a piece of chalk in vinegar, 

 or in any of the stronger acids of the school laboratory, 

 all the limestones are peculiarly susceptible to this form 

 of chemical attack. Peat -water eats into limestone 

 rapidly, while the limestone above the stream escapes, 

 though it is a little (and much more slowly) dissolved by 

 rain. Hence arise some curious features in the scenery 

 of limestone districts. The walls of limestone above the 

 water are not eaten away so fast as their base over which 

 the water flows. Consequently they are undermined and 

 are sometimes cut into tunnels and caverns and caves. 



The rivers carry away the dissolved material. The 

 carbonate of lime is taken to the sea ; and this substance, 

 of which sea shells, for example, are principally formed, is 

 constantly supplied to the sea by the rivers that trans- 

 port it from the land. The rivers of Western Europe 

 have been known to convey one part of dissolved mineral 

 matter in every 5000 parts of water, and of this mineral 

 matter one half is carbonate of lime. The Rhine alone 

 bears enough carbonate of lime to the sea every year to 

 make 332,000,000,000 oyster shells of the usual size. The 

 Thames conveys 180,000 tons of sulphate of lime past 

 London every year. It has been computed that more 

 than 8,000,000 tons of dissolved mineral matter are re- 



48 



