GROUND-WATER 41 



(and some plants) of the sea for making their shells, tests, bones, 

 etc., and these are left on the sea bottom when the organisms die. 



Deposition. After dissolving mineral matter from the rocks, 

 ground-water sometimes leaves a part of it in the pores and cracks 

 of the rock through which it flows. In this way it tends to fill up 



Fig. 26. A quartz vein (the white band) in contorted schist. Muchals 

 Caves, Kincardineshire, Scotland. 



cracks. When cracks in the rocks are filled or partly filled by min- 

 eral matter deposited from solution, they become veins (Fig. 26), 

 and some rocks are full of them. Ores of gold, silver, lead, zinc, etc., 

 occur in some veins. 



The mineral matter dissolved in ground-water is often brought 

 to the surface and deposited there. The deposition is brought 

 about in various ways, among which are the following: (1) When 

 water evaporates, the mineral matter dissolved in it is left behind. 

 This is one of the reasons why kettles in which water is boiled be- 

 come coated with mineral matter. (2) Certain gases dissolved in 

 water help it to dissolve mineral matter. If water contains much 

 gas, and if the gas escapes, as it is likely to when it is heated or when 



