232 



ROCKS AND SOIL 



Erosion by Solution. You know that sugar placed in 

 lemonade soon dissolves in it and disappears, but the 

 sugar may be tasted in all parts of the glass of lemonade. 

 The sugar passes from the solid to the liquid state by a 

 physical process known as solution. If a substance dis- 

 solves in a liquid completely, we say the substance is 

 soluble and that we have a solution. Rain water, although 

 perfectly pure as it starts to fall from the clouds, dissolves 

 air in falling and after soaking into the ground may soon 

 take up mineral matter into solution. As we have seen, 

 certain minerals, particularly the compounds of calcium 

 and magnesium, are slightly soluble in water which con- 

 tains CO 2 . In some parts of the country, great gaps and 



caves have been 

 formed underground 

 where limestone has 

 been dissolved and 

 removed by running 

 water. 



Hot Springs and 

 Geysers. Under- 

 ground water which 

 comes to the surface 

 in springs may con- 

 tain mineral matter 

 in solution. The 

 mineral springs of the 

 United States are 

 valued at millions of 

 dollars. Many of 

 these waters are 

 claimed to have me- 

 dicinal properties. 

 In regions where vol- 



Old Faithful is so called because it always erupts . . 



on time. Can you find out what causes this action? CaniC action has been 



