CHANGE OF VOLUME BY RECRYSTALLIZATION. 101 



resulting from pressure as the chief motive force; and it may be that 

 chemical interchange is one of the processes which explain the consolidation 

 of the clay in Spring's experiment. 



In the rocks a smaller volume may result in either of two ways: 

 Material may be taken into solution and deposited in a more compact 

 form without change in chemical composition, or with change in chemical 

 composition. 



ROCK VOLUME AS THE RESULT OF SOLUTION AND DEPOSITION WITHOUT CHANGE IN CHEMICAL COMPOSITION", 



It has already been explained (pp. 77-78) that pressure promotes solu- 

 tion in case the volume of the solution is less than that of the solvent and 

 solid, and that pressure promotes precipitation in case the volume of the 

 solution is greater than that of the solvent and solid. Thus the solubility 

 of a salt increases with pressure, provided the dissolving is associated with 

 a contraction of the volume of the solution plus the salt; and, conversely, 

 the solubility decreases if the separation of the salt (from the solution) is 

 associated with a diminution of the volume of the system. 01 In ground 

 solutions the general law is that the volume of the solution is less than that 

 of the substances dissolved and the water. It follows from this law that 

 pressure in rocks, the interstices of which are filled with water, promotes 

 recrystallization and condensation. 



The production of a smaller rock volume without change in chemical 

 composition may occur where the recrystallization and condensation take 

 place without change of minerals, and where the recrystallization and 

 consolidation take place with change of minerals. 



Recrystallization and condensation without change of minerals. As aU illustration of the 



principle, we may consider a stratum of unconsolidated crystallized calcium 

 carbonate over which is a layer of water saturated with calcium carbonate. 

 Inasmuch as the calcium carbonate is porous, the water in the rock is free 

 to move and is under the pressure of the hydrostatic column above it. 

 The particles of CaC0 3 are under this pressure, and also that of the solid 

 above All the water in the crevices and pores small enough to hold water 

 by capillarity is under both the pressure of the water and in part that of 

 the rock. This water is saturated under this pressure, and it can hold more 

 substances in solution than the water under less pressure. An interchange 



«Nemst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York, 1895, 

 p. 567. 



