38 



PHYSIOGRAPHY 



Chemical Work 



Solution. All water which comes out of the ground has some 

 mineral matter in solution. This has been dissolved from the rock 

 through which the water has passed. When spring or well water 

 is evaporated, and often when it is boiled, it leaves a little residue, 

 which in time becomes very noticeable, as in boilers and kettles. 

 Thus the coating on the inside of a tea-kettle is composed of mineral 

 matter which was in solution in the water, and which was left behind 

 when the water was heated or evaporated. 



Pure water does not dissolve mineral matter readily; but ground- 

 water is not pure, for in falling through the atmosphere it dissolves 

 gases from the air, and in sinking through the soil it takes up the 

 products of plant decay. With these impurities in solution, ground- 

 water dissolves most sorts of mineral matter more readily than 

 pure water would. The amount of mineral matter brought to the 

 surface through springs is very great. Thus the springs of Leuk 

 (Switzerland) have been estimated to bring to the surface more than 

 2,000 tons of gypsum in solution yearly. 



Caverns. By the dissolving work of ground-water, rock is 



made porous. Small pores 

 and cavities are more numer- 

 ous than large ones, but some 

 of the openings produced in 

 this way, such as Wyandotte 

 Cave in southern Indiana, and 

 Mammoth Cave in Kentucky, 

 are very large. Such caves 

 occur chiefly in limestone, for 

 this is the most soluble of the 

 common rocks. An under- 

 ground cave is not, as a rule, 

 one great chamber, but is 

 made up of many chambers 

 or rooms connected by small- 

 er passageways (Fig. 24). The total length of the passageways 

 in Wyandotte Cave is more than 23 miles, and it has been 



Fig. 24. Vertical section of a cave in 

 France. B, a chamber without 

 outlet; C, D, E, F, G, H, cone- 

 shaped chambers. (Robin.) 



