THE WORK OF GROUND-WATER. 223 



from the rock. Thus the process of substitution is effected. By this 

 process the Hme carbonate of a shell imbedded in rock may be removed, 

 molecule by molecule, and some other substance, such as siUca, left 

 in its place. When the process is complete, the substance of the shell 

 has been completely removed, though its form and structure are still 

 preserved in the new material which has taken the place of the old. 

 Buried logs are sometimes converted into stone by the substitution 

 of mineral matter for the vegetable tissue. This is petrification. Petri- 

 fication is altogether distinct from incrustation, which simply means 

 the coating of an object with mineral matter. A bird's nest may be 

 incrusted with Hme carbonate, but it is not thereby petrified. Solu- 

 tion is a necessary antecedent of substitution, 



3. The materials which are subtracted from the rock at one point 

 may be added to other rock elsewhere. Thus a third type of change, 

 addition, is effected. Rock may at one time and place be rendered 

 porous by the subtraction of some of its substance, and the openings 

 thus formed may subsequently become the receptacles of deposits 

 from solution. This is exemplified in the stalactitic deposits of many 

 caves. Not uncommonly cracks and fissures are filled with mineral mat- 

 ter deposited by the waters which pass through them. Thus arise 

 veins which, for the most part, are nothing more than cracks and crevices 

 filled by mineral matter brought to them in solution, and precipitated 

 on their walls. Most veins of metallic ares have originated in this way. 



4. A further series of changes is effected by ground-water when it, 

 or the mineral matter it contains, enters into combination with the 

 mineral matter through which it passes. One of the commonest pro- 

 cesses of this sort, hydration, has already been referred to (pp. 43 and 428) ; 

 but in the development of many of the commoner hydrous minerals 

 changes other than hydration are involved. These changes result in new 

 mineral combinations, the new minerals being developed out of the old, 

 usually with some additions or subtractions. In the long course of 

 time changes of this sort may be very great, so great indeed that large 

 bodies of rock are radically changed, both chemically and physically. 

 Much of the old substance may remain, but it has entered into new 

 and more stable combinations with the materials which the water has 

 brought to it. 



Quantitative importance of solution. — In general, solution is 

 probably most effective at a relatively sHght distance below the sur- 



