124 PHYSIOGRAPHY. [chap. 



cavern, examples of these stalactites may readily be seen. 

 It is, in fact by no means uncommon to see small stalactites 

 hanging down, like icicles, from the roof of the arches of 

 a railway bridge, where they are produced by the rain-water 

 dissolving the calcareous matter contained in the roadway, 

 or in the materials of which the arch is composed. 



Calcareous salts, although the most common, are by no 

 means the only mineral compounds which occur in natural 

 waters. Some springs, such as those at Epsom, are rich in 

 sulphate of magnesia ; whence this salt is popularly called 

 Epsom salts, while the springs themselves are said to be 

 saline. Others may contain salts of iron, and form chalybeate 

 springs, as mentioned at p. 26. It is notable that many 

 mineral springs have a temperature higher than that of the 

 locality in which they issue ; thus the warm springs of Bath 

 have a temperature of nearly 120° F. In volcanic districts, 

 such thermal sources are extremely common, and as water 

 when hot dissolves most substances more freely than when 

 cold, these springs are often rich in mineral matter. The 

 famous geysers of Iceland and of Colorado are intermittent 

 boiling springs, containing in solution a good deal of silica, 

 or the matter of which flint and rock crystal are composed. 

 (See Chapter XIII.) 



Springs, such as have been referred to above, are of course 

 exceptional ; but, it should be remembered, that all spring 

 water contains more or less mineral matter in solution. 

 On comparing the composition of rh<er water with that of 

 spring water, it will generally be found that the river con- 

 tains less saline matter. In fact, the water discharged into 

 the river by springs becomes diluted by direct influx of 

 rain, and this dilution more than compensates for loss by 

 evaporation ; so that, on the whole, the proportion of salts 

 diminishes. Moreover, the organisms inhabiting the river 



