ACTION OF AQUEOUS SOLUTIONS. 65 



tions and the solids, but that in this film the migration of the dissolved 

 material is to some extent stayed by the molecular attraction of the 

 crystals. 



Aqueous solutions as a geological agent require consideration from two 

 points of view — the chemical and physical principles controlling the action 

 of ground water, and the circulation and geological work of ground water. 

 These are treated in the following sections I and II, respectively: 



SECTION I. CHEMICAL AND PHYSICAL PRINCIPLES CONTROLLING THE ACTION 

 OF GROUND WATER. 



The work of ground water, like any other work, requires the expendi- 

 ture of energy. The energy by which the water accomplishes its work is 

 derived from chemical action, heat, and mechanical ; 'tion. 



In order to comprehend the processes of alteration of rocks it will be 

 necessary to summarize the important conclusions of physical chemistry as 

 to solutions and chemical reactions. The principles here contained are 

 mainly taken from the works of Ostwald and Nernst. 



Chemical action will be considered under the headings, " Principles of 

 solutions applicable to ground waters," and "Principles of chemical reactions 

 applicable to ground waters." 



PRINCIPLES OF SOLUTIONS APPLICABLE TO GROUND WATERS. 



While the consideration of the principles of solution logically falls 

 under general chemical action, and, perhaps, ought to be treated as a special 

 case under the general treatment of chemical reactions, it seems advisable, 

 because the subject of solutions is somewhat simple as compared with the 

 interactions of complex chemical compounds, to take up this subject first, 

 after which the general laws controlling chemical reactions will be given. 



The water of rocks, whether at ordinary temperatures and pressures or 

 at higher temperatures and pressures, may take any of the substances 

 with which it comes in contact into solution; it may deposit substances 

 from solution; it may combine with substances forming hydroxides, as in the 

 case of many of the zeolites and limonite; it may part with its hydrogen in 

 exchange for bases, thus at the same time changing the composition of the 

 rock and taking the bases replaced into solution. This is illustrated by the 

 alteration of enstatite to talc. (See Chapter V, p. 268.) There may be 



MON XLVII-'Ott 5 



