64 A TREATISE ON METAMORPHISM. 



In the belt of weathering, above the free surface of ground water, 

 gaseous solutions and liquid solutions work together. In this belt the rocks 

 are not ordinarily saturated with water, but on the average contain a con- 

 siderable amount of water held by adhesion between the liquid and the solid 

 mineral particles. It is believed that in this belt the gases act upon the 

 rocks chiefly through water solutions. As evidence of this is the small 

 amount of decomposition of the disintegrated rocks in arid regions. (See 

 pp. 496-498.) It therefore appears that the dominant agents of alterations 

 in the belt of weathering are aqueous solutions. 



In the belt of cementation below the free surface of ground water the 

 rocks are practically saturated, and in this belt aqueous solutions are the 

 chief agents of alterations. 



Water solutions are also a chief agent in the transportation of material 

 from one place to another. 



At this point it is necessary to understand that the places of interaction 

 of aqueous solutions and solids are the contacts between the two. It will 

 be seen later that, on account of the molecular attraction between water and 

 rock, a thin film of water adheres to the solid particles with which it is in 

 contact. This film is not in active circulation, yet it is the part of the 

 agent, water, which is immediately concerned in the transfer of mineral 

 material from the rocks to the solutions and from the solutions to the rocks. 



The contact film may take material of the rock into solution. From 

 this film the materials taken into solution migrate to other parts of the 

 solution. Probably the migration from the contact film to the free water is 

 largely by diffusion (see pp. 82-83) ; but, once beyond the contact film, the 

 migration is largely accomplished by convectional movements. Material 

 may be supplied to the contact film by migration of material from the free 

 parts of the solution. From the contact film material may be deposited in 

 the rocks. 



In this connection it is interesting to note that in the portions of the 

 solutions near the contact with solids "there is often a concentration of 

 the dissolved material. This phenomenon has been called adsorption."" 

 The phenomena of adsorption seem to show with great clearness, not only 

 that the contact film is the active agent in transfer between the free solu- 



« Cameron, Frank K., Application of the theory of solutions to the study of soils: Report No. 64, 

 Field Operations of Division of Soils, 1899, U. S. Dept. of Agric, 1900, p. 142. 



