ATMOSPHERIC AGENCIES. 



Fig. 2. 



changed into soil, the soil is also slowly carried away by agencies to be 

 hereafter considered ; and these changes, taking place more rapidly in 

 some places than in others, give rise to a great variety of forms, some 

 of which are represented in the accompanying figure (Fig. 2). 



In the process of disintegra- 



l ^SLl c 



Fig. 3. — a, vegetal soil : b. mineral soil ; c. harder 

 portions of rock left in process of disintegra- 

 tion; d, underlying rock. 



Fig. 4. 



tion the original blocks lose their 

 prismatic form, and become more 

 or less rounded, and are then called 

 boivlders of disintegration. These 

 may lie on the surface (Fig. 2), or 

 may be buried in the soil (Fig. 3). 

 When of great size and very solid, 

 so as to resist decomposition to a greater extent than the surrounding 

 rocks, they often form huge 

 rocking -stones (Fig. 4). These 

 must not be confounded with 

 true bowlders and rocking- 

 stones which are brought from 

 a distance, by agencies which 

 we will discuss hereafter, and 

 which are, therefore, entirely different from the subjacent or coun- 

 try rock. 



General Explanation. — The process of rock-disintegration may be 

 explained, in a general way, as follows : Almost all rocks are composed 

 partly of insoluble materials, and partly of materials which are slowly 

 dissolved by atmospheric water. In the process of time, therefore, 

 these latter are dissolved out, and the rock crumbles into an insoluble 

 dust, more or less saturated with water holding in solution the soluble 

 ingredients. To illustrate : common hardened mortar may be regarded 

 as artificial stone ; it consists of carbonate of lime and sand ; the car- 

 bonate of lime is soluble in water containing carbonic acid (atmospheric 

 water), while the sand is quite insoluble. If, therefore, such mortar be 

 exposed to the air, it eventually crumblec into sand, moistened with 

 water containing lime in solution. Again, to take a case which often 

 occurs in Nature, it is not uncommon to find rock through which iron 

 pyrites, FeS 2 , is abundantly disseminated. This mineral is insoluble ; 

 but under the influence of water containing oxygen (atmospheric water) 

 it is slowly oxidized and changed into sulphate of iron, or copperas^ 

 which, being soluble, is washed out, and the rock crumbles into an 



