Geology 



and some of the rocks in the bed of the stream were also 

 coated with similar material. This tufa consisted of 

 Calcium carbonate (CaCO 3 ), and was evidently deposited 

 from solution in the water of the river, which had dis- 

 solved portions of the limestone in and over which it 

 had flowed. 



The solution of limestone is possible owing to the 

 presence in the atmosphere, and in consequence in rain 

 water, of Carbonic acid (CO 2 ). Carbonic acid in the 

 presence of water unites with Calcium carbonate to form 

 a body which is soluble in water, but is very unstable, 

 thus : CaCO 3 + CO 2 + H 2 O = H 2 Ca(CO 8 ) 2 . 



There is an equilibrium between these various sub- 

 stances in the solution, and anything which tends to 

 disturb this equilibrium, for example changes of tempera- 

 ture or of pressure, will tend to cause a further solution 

 of calcium carbonate, if such be available, or a deposition 

 of the same substance, should the change be in the 

 opposite direction. Thus when a solution of calcium 

 carbonate, containing as much of the solid as it is 

 capable of holding, is spread out in a thin sheet, or 

 broken into spray as in a waterfall, or falls drop by 

 drop from the roof of a cave, some of its carbonic acid 

 escapes. A corresponding amount of calcium carbonate 

 is deposited in the solid form, as a crust on the rock 

 surface over which the water is spread out, as an apron 

 of tufa on the face of the waterfall, or ^as stalactites on 

 the roof of the cave. 



All the stream and spring waters of this limestone 

 area were saturated with calcium carbonate, and conse- 

 quently deposits of calcareous tufa were extremely 

 common. One of the most interesting was a deposit 



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