HISTORY OF LANDSCAPE 187 



Hard water, if not too hard, is pleasanter for drinking than soft, 

 and has the advantage that it does not corrode lead or iron pipes 

 so readily as soft water does. But it is not so good for most 

 manufacturing purposes, such as dyeing, or for steam raising: 

 Consequently, it is often softened wholly or partially by water 

 companies. Permanently hard waters are especially valuable 

 for brewing. 



A temporarily hard water, if it contains very much carbonate 

 of lime in solution, may be compelled to deposit some of this 

 substance in consequence of its natural loss of carbonic acid, 

 when the spring first reaches the open air. This is the origin 

 of petrifying springs. The " turning into stone " of objects 

 placed in these springs is merely an incrustation of lime thrown 

 down upon them. The objects usually employed for petrifaction, 

 like moss and birds' nests, are such as break the water up and 

 encourage rapid evaporation of the carbonic acid. Sometimes 

 springs deposit enough carbonate of lime to form considerable 

 rock masses, and in Rome " travertine " thus deposited is largely 

 used as a building stone. 



The dissolved material carried by springs is taken out of the 

 rocks traversed, and therefore springs are powerful disintegrating 

 and transporting agencies. Often, and especially in limestone dis- 

 tricts, they dissolve the sides of the cracks along which they 

 travel, widening them out into systems of caves. These are often 

 so important that they carry the whole of the drainage of the 

 area along underground channels, which are often of considerable 

 complexity. Old valley systems formed in earlier days, before the 

 water had opened out its underground channels, are often left 

 dry above (Fig. 74), or are only occupied by water during seasons 

 of heavy rainfall, when the rock below is saturated and its cracks 

 filled with water. Eventually the caves fall in and a new surface 

 drainage exists for a while, until the water has found a still lower 

 set of channels through the rocks. 



Even when caves are not formed, solution of the cement 

 of a compact rock disintegrates it and prepares it for removal 

 by running water. The consequences are even more serious 

 when the water travels along the junction of two rocks and 

 removes the cement from the upper of them. If the junction crops 



