DENUDATION 99 



freezing and thawing a moistened porous rock like sandstone 

 might be demonstrated experimentally. Disintegration of this 

 sort does not affect the composition of the rock, merely its state 

 of aggregation, and it is hence called mechanical disintegration. 



As an example of chemical disintegration, it is best to take 

 a sandstone or conglomerate, cemented by carbonate of lime, and 

 submit it to the action of dilute hydrochloric acid. This will 

 demonstrate that the rock consists of insoluble sand or pebbles, 

 united by a cement soluble in the acid. Similar action on a 

 limestone will show that the insoluble portion is insignificant, and 

 the soluble constituent is in excess. Hydrochloric acid is in 

 nature replaced by carbonic acid, which is washed out of the air 

 by rain and held in solution by it. In the field the surface of a 

 sandstone will often prove to be converted into loose, uncemented 

 sand, the usual cement, carbonate of lime, being susceptible to 

 solution by rain-water containing carbonic acid. The exposed 

 part of a conglomerate will pass into a loose, pebbly gravel under 

 the same influence. If a limestone is similarly exposed it will be 

 found to be encrusted or replaced by clay or rotten-stone, the 

 former the insoluble argillaceous ingredient of some limestones, 

 the latter the siliceous ingredient of others. If the limestone is 

 a chalk with nodules of flint, the flint will be left behind and the 

 chalk dissolved. 



Even much harder rocks than these are attacked by carbonic 

 acid. Granite, basalt, and other volcanic rocks are found to have 

 some of their ingredients, and especially their most important 

 one, felspar, decomposed by this weak acid, some of the con- 

 stituents being dissolved and carried away in solution, and others 

 left behind in a soft condition resembling clay. The effect on the 

 rock is like that of removing cement. 



Thus hard and solid rocks are found to be broken up by either 

 mechanical or chemical disintegration, and the resultant material 

 is left ready to be removed, while the process of removal is rendered 

 very much easier than if the unaltered rock had to be dealt with. 

 This second process, the removal of material, is known as transport, 

 and disintegration and transport together make up the process 

 of denudation by which the surface of the earth's crust is being 

 continually changed. 



