160 DISINTEGRATION OF ROCKS. 



example, of shells, is often entirely removed and re- 

 placed by carbonate of iron, pyrites, silex, or some 

 Other ingredient, such as mineral waters often con- 

 tain in solution. It rarely happens, except in the 

 limestone rocks, that the carbonic acid can dissolve 

 all the constituent parts of the mass : and for this 

 reason, probably, calcareous rocks are almost the 

 only ones in which great caverns and long winding 

 passages are found." 



De la Beche* observes, that " rocks receive consid- 

 erable chemical modification by the percolation of 

 water through them. There is scarcely any spring- 

 water which does not contain some mineral substan- 

 ces in solution, which it mi** t have procured in its 

 passage through the rocks. Now, though this quan- 

 tity may be small when we regard the composition 

 of any particular spring- water, yet, when we consider 

 the soluble matter contained in the spring-waters of 

 any given 1000 square miles of country, and that 

 this subtraction of matter from rocks has been go- 

 ing on for ages, we may readily conceive that the 

 chemical change may be greater than, at first sight, 

 we might anticipate. We may also infer that the 

 more soluble portions of rock have a constant ten- 

 dency to be removed when exposed, not only to di- 

 rect atmospheric influences, but also to the perco- 

 lation of rain-water through them, so that most 

 rocks would experience great difficulty in resisting 

 chemical changes of this kind, and of preserving 

 their original chemical nature, more particularly 

 when elevated into the atmosphere." 



In proof of the correctness of the opinion that 

 .water dissolves some of the elements and holds 

 them in solution, we may refer to the large quanti- 

 ties of lime contained in the water of limestone 

 countries, communicating the character of hardness 

 to the water; and shown also by its deposition on 



* Researches in Theoretic Geology, p. 86. 



