%5Q iDISINTEGRATION 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 must 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. 
