14 DYNAMICAL GEOLOGY. 



these now. But in the Southern States also, in many 

 places, are found huge, isolated masses, lying on the sur- 

 face, and even sometimes forming rocking stones (Fig. 4). 

 If we examine these, we find that they are of the same 



PIG. 4. 



material as the country-rock. They have been formed in 

 place. In the general disintegration of rock, and forma- 

 tion and removal of soil, these have resisted, because 

 harder than the rest. Nothing is more interesting than 

 thus to trace the configuration of the surface of the 

 country to unequal resistance to atmospheric agencies. 



Explanation of Rock-Disintegration. If we take a 

 piece of old and very hard mortar, and pour on it a little 

 hydrochloric acid, it quickly breaks down into sand, wet 

 with a solution of calcium chloride. The explanation is 

 simple. Mortar consists of grains of sand cemented into 

 a mass by hydrate or carbonate of lime. The acid dis- 

 solves the lime-cement, and the mass falls to powder. 

 Now, mortar is really artificial stone, and nearly all rock is 

 constituted in a similar manner, i. e., consists of particles 

 cemented together. In all rock some parts are soluble in 

 atmospheric water, and some are not. Under the long- 

 continued action of this agent, therefore, the soluble 

 parts are dissolved, and the mass breaks down into a 

 powder, or dust of the insoluble parts, wet with a solu- 

 tion of the soluble parts. The main difference between 

 the experimental and the natural case is, that in one the 

 process is rapid, and in the other extremely slow. 



Examples. One or two examples will make this plain : 





