GROUND-WATER 203 



shell imbedded in rock may be removed, molecule by molecule, 

 and some other substance, such as silica, left in its place. When 

 the process is complete, the substance of the shell has been com- 

 pletely 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 petrifaction (Fig. 

 167). Solution 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 matter deposited by the waters which 

 pass through them. Thus arise veins (Fig. 20) which, for the 

 most part, are cracks and crevices filled by mineral matter brought 

 to them in solution, and precipitated on their walls. Most veins 

 of metallic ores 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 processes of this sort, hydration, has already been 

 referred to (p. 102) ; but in the development of many of the com- 

 moner 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 so great as to change rock radically, both chemically and 

 physically. 



Quantitative importance of solution. In general, solution is 

 probably most effective at a relatively slight distance below the 

 surface. In the mantle rock, the materials are usually less soluble 

 than below, for in many places they represent the residuum after 

 the soluble parts of the formation from which they originated were 

 dissolved out. Below this zone the rock contains more soluble 



