102 A TREATISE ON METAMORPHISM. 



is constantly carried on between the free and the capillary water, and as the 

 capillary water becomes free it is supersaturated and deposits some of its 

 load in the interstices of the rock. But gravity ever pulls the material 

 downward, and although this process is not rapid, it is continuous, and in 

 course of time the particles are cemented. A solidified and recrystallized 

 limestone is produced. Evidently the greater the pressure the more rapid 

 and complete is this change. 



Another example of solidification without change in mineral composi- 

 tion is the change of snow or separate ice crystals where mingled with 

 water to solid ice, as at the head of glaciers. Ice has its melting point low- 

 ered by pressure. Where the granules are under more than the average 

 pressure some of them melt. The water flows out into the free spaces and 

 is again frozen. Or, as expressed above, under more pressure more of the 

 ice is dissolved in the water than under less pressure. When the pressure 

 is relieved in the more open spaces the ice is reprecipitated." As the 

 process goes on the particles are finally cemented. This process, like that 

 of the recrystallization of limestone, is continuous, and finally the separated 

 snow granules are transformed to continuous ice. 



Recrystallization and condensation with change of minerals ReCl'yStallizatioU and COU" 



densation with change of minerals but without chang-e in chemical composi- 

 tion may take place by precisely the same processes as already given. The 

 resultant minerals, where the inducing cause is pressure, are more compact 

 than the original minerals. Illustrating this principle, pressure induces 

 the transformation of amorphous calcium carbonate to calcite. Similarly, 

 pressure may induce the transformation of many other amorphous substances 

 to crystalline forms. Pressure also induces minerals to change to forms 

 having higher specific gravities. Thus pressure tends to transform tridyrhite, 

 ;sp. gr. 2.28-2.33, to quartz, sp. gr. 2.653-2.660; and marcasite, sp. gr. 

 4.85-4.90, to pyrite, sp. gr. 4.95-5.10. (See pp. 220-221, 215.) 



We may also safely argue that, where the pressure is great, minerals 

 are not likely to crystallize in forms having low specific gravities. Thus 

 under great pressure it is to be expected that silica will crystallize as 

 quartz and not as tridymite. Doubtless this principle explains why 

 quartz is always found in the plutonic rocks, and why tridymite often is 



«Le Chatelier, in Theoretical chemistry, by W. Nernst, p. 654. Zeitschr. phys. Chemie, vol. 9, 

 1892, p. 335. 



